2.1. A positivist methodology

As rightly emphasized by Corten, the divergences in the legal literature on the use of force are often due to the different methodologies followed by scholars. Although acknowledging the difficulty in classifying all the doctrinal opinions as applying one or another specific methodology, Corten distinguishes two main kinds of approach generally used by scholars: the extensive versus the restrictive approach. He explains this distinction as follows.Footnote ⁶ The extensive approach, generally used by the ‘current predominant in the English-speaking world that favours a “policy-oriented” perspective’,Footnote ⁷ considers customary law as a privileged means of adapting international law, since it is more flexible and enables one to take into account values and other subjective factors in the evolution of a norm. This approach focuses on the practice element of custom and ascribes a particular weight to the decisions adopted by political organs such as the UN Security Council. Moreover, it does not exclude that custom be established on a relatively small number of cases and be inferred from the practice of only leading, democratic, and/or powerful states. Such an approach unsurprisingly makes it easier to conclude that contemporary law on the use of force has evolved. In contrast, the restrictive approach, generally used by scholars belonging to the positivist tradition and favouring a formalistic or voluntarist view of international law,Footnote ⁸ places treaties and custom on an equal footing. As far as the formation of customary law is concerned, this approach pays particular attention to the opinio juris element, particularly to clear legal declarations, especially those made by states in international fora such as the UN General Assembly. Moreover, according to this approach, evolutions of the customary law on the use of force are only possible if based on the repetition of numerous similar precedents, evidencing that all states have accepted such evolutions. The legal regime on inter-state use of force having a peremptory nature, any change of such a regime implies a ‘near-unanimous’ agreement among states. As a result, this approach makes it much harder to assert that the contemporary law on the use of force has evolved.

While not dismissing the extensive approach as irrelevant or unfounded, Corten clearly states at the beginning of his book that he will adopt the method applied by the International Court of Justice (ICJ), which, according to him, follows a restrictive approach with regard to the evolution of the customary law on the use of force.Footnote ⁹ In his view, any change in this law therefore requires, first, that a state invokes a new right and, second, that such a new right is accepted by the other states. The first requirement implies that, when using force, a state makes a claim formulated in legal terms and bearing on the evolution of the legal rule and not on the interpretation of facts. The second requirement involves the claimed evolution being accepted, preferably through express and clear approval rather than silence or tacit acceptance, and that such approval bears on the modification of the legal rule and is expressed by the international community as a whole.Footnote ¹⁰ Corten emphasizes in this respect the value of the general UNGA resolutions on the use of force – such as Resolution 3314 (XXIX) – as well as the debates preceding those resolutions, since both evidence clear official and legal positions of principle of a large number of states.

This methodological part is undoubtedly a crucial part of the book. Conclusions reached by the author on the contemporary law on the use of force directly ensue from the restrictive methodology that he has chosen to follow throughout his analysis of the matter. Therefore, particular attention will be paid to this first part. Two general comments will be made in that respect. A first comment is that the book reproduces almost entirely the French version, including the methodology part. This certainly has the merit of introducing into the broad English-speaking legal sphere, which is generally more sensitive to policy-oriented arguments, another way of thinking about the matter, particularly a strict positivist reasoning that is typical of French legal scholars. It is nonetheless difficult to determine in advance what impact the book will have in the English-speaking literature, especially among US scholars. Indeed, as carefully emphasized by the author himself, his conclusions are only relevant if one follows a restrictive method. Basically, another method, like a more policy-oriented approach, is perfectly arguable even if it leads to an entirely different conclusion. The risk is, then, that the book will be seen, at least by American scholars, as an unidentified flying object – that is, as an object that will be irrelevant to comment upon if those scholars adopt their proper methodological approach while at the same time difficult to discuss in the light of Corten's method since they are unfamiliar with such a method. More fundamentally, one may wonder why, although sometimes suggesting it,Footnote ¹¹ Corten did not expressly contend that his methodology was the most valid. His approach suggests that international law is twofold and may be divided into one international-law regime applied by US scholars and another applied by positivist Continental lawyers. Yet, the ICJ – just to mention one example – does not apply two different kinds of international law when ruling on a case. Corten could perfectly argue against the extensive approach by invoking persuasive arguments, such as those put forward by scholars criticizing the inconsistent results of this approach.Footnote ¹²

A second comment regarding Corten's distinction between the extensive and restrictive approaches concerns the elements upon which he bases this distinction. In his view, proponents of the extensive approach privilege state practice as they focus on state material conduct whereas proponents of the restrictive approach privilege the opinio juris of states as they focus on state legal declarations. This assertion is debatable for two main reasons. First, it seems difficult to equate state material conduct with state practice and state declarations with opinio juris. State material conduct and state declarations are, indeed, two types of state practice. The latter, like the former, are distinct from opinio juris, which is an abstract element requiring that state practice be followed out of a belief that it conforms to law. Therefore, it seems technically more accurate to consider that proponents of both approaches distinguish themselves by the fact that they focus on different types of state practice, state material conducts versus declarations, as a means for revealing the opinio juris of states. Second, it is not sure as a matter of fact that scholars following the extensive approach only focus on state material conduct, which seems logical, since such conduct is itself meaningless.Footnote ¹³ Legal literature shows that, while putting emphasis on state material conduct, those scholars actually interpret such conduct in the light of other elements, including state – political, moral, historical, or even legal – declarations, those elements being sometimes very decisive in the interpretation of the case under review.Footnote ¹⁴ In sum, although the final aim of proponents of both approaches is to identify the opinio juris of states on the basis of their practice, those favouring an extensive approach distinguish themselves from the others to the extent that they are ready to rely on any state manifestation, including only material conduct if no other practice exists, in order to identify the opinio juris that is, in reality and not just officially, in the mind of the intervening states. The ultimate objective of such an approach seems to ‘stick’ international law as much as possible to the reality and to avoid asserting a legal discourse that would not correspond to the actual conducts of states.Footnote ¹⁵

That being said, showing, as Corten does, that different methods are followed in legal literature on the use of force significantly helps us to understand the profoundly different conclusions that are often reached in that matter as well as to be aware that those conclusions are actually not conflicting, since they ensue from dissimilar methods. This should hopefully put an end to the frequent ‘dialogues of the deaf’Footnote ¹⁶ appearing in the legal literature as well as in conferences on the use of force.

2.2. A restrictive positivist methodology

Corten's method is a restrictive method, typical of the legal positivist tradition. It is logically stricter than the extensive approach. One may nonetheless wonder whether it is not particularly restrictive even from a pure positivist perspective due to some explicit (sub-subsection 2.2.1) as well as implicit (sub-subsection 2.2.2) methodological choices that Corten seems to have made in his book.

3.1. Use of force

As far as the notion of use of force is concerned, Corten's main contribution is to demonstrate the existence of a distinction between use of force and police measures as well as to identify in great detail the criteria upon which such a distinction may be drawn in practice.Footnote ³⁹ This original distinction is particularly relevant and very helpful. It enables one to avoid any cross reflections, which would confuse two different conceptual categories being regulated by two distinct regimes. It also enables Corten to show that, although the law on the use of force must be strictly conceived as not allowing any other exceptions than the traditionally admitted ones, a more flexible approach is nonetheless possible with respect to the right to take extraterritorial police measures. Indeed, circumstances precluding wrongfulness cannot be invoked to justify any resort to force, since, as will be argued by the author,Footnote ⁴⁰ the whole prohibition on the use of force has a peremptory nature. Such circumstances could nevertheless serve as a means for excluding the wrongfulness resulting from the undertaking of police measures carried out in the territory of a state without its consent, since the rule allegedly infringed in this case would merely be the respect for the sovereignty of that state. In other words, it is thanks to such an original distinction that Corten may coherently argue for a restrictive view on the use of force, a ‘law against war’, whose scope does not, however, encompass the more flexible regime regarding police measures.

Such a distinction is founded upon an in-depth scrutiny of state practice and international case law. Corten addresses the three levels at which it appears: on land,Footnote ⁴¹ at sea,Footnote ⁴² and in the air.Footnote ⁴³ It is on the basis of a similar comprehensive analysis that he identifies the two main criteria for qualifying a force as a military force or a police measure: the gravity of the forceFootnote ⁴⁴ and the intention behind this force.Footnote ⁴⁵ Many examples are given in order to support this conclusion. For instance, the minor gravity of the force used by the Israeli commandos in order to abduct Adolf Eichmann from Argentina in 1960 is indicative of the police nature of this action. By contrast, the significant gravity of the US military operation in Panama, in 1989, notably aimed at abducting General Noriega, clearly reveals the use-of-force nature of this operation.Footnote ⁴⁶ It is in light of this distinction and these criteria that Corten addresses the more debated question of the qualification of ‘targeted’ military operations,Footnote ⁴⁷ encompassing all limited coercive actions undertaken in a foreign territory, such as the ‘pursuit of an armed band of criminals who ha[ve] crossed the frontier’.Footnote ⁴⁸ He then completes his analysis by summarizing a series of questions to which the answers are intended to facilitate the assessment of the two above-mentioned criteria in a particular case. Those questions relate to the place where the coercive action was taking place, the context in which it occurred, the persons who decided to undertake and carry it out, the target thereof, the issue of whether it has given rise to confrontation between the agents of two states, and the scope of the means implemented by the intervening state.Footnote ⁴⁹

Such considerations, especially those related to the gravity and intention criteria, raise several remarks. The gravity criterion is undoubtedly decisive for assessing whether an armed (or military)Footnote ⁵⁰ force is a use of force within the meaning of Article 2(4) of the UN Charter or a police measure. In identifying the threshold under which a coercive action must be considered as a police measure, Corten nonetheless appears to include precedents or examples that do not seem to involve any armed force at all in the sense in which this notion is commonly understood. In his reasoning, he analyses the passages of foreign military aircraft or missiles in the airspace of a state as not reaching the threshold necessary for being qualified as a use of force.Footnote ⁵¹ He reaches a similar conclusion with respect to the hypothetical situation in which ‘soldiers cross the border illegally to sunbathe on the beach on another state's territory’.Footnote ⁵² As entailing no physical violence, causing or intended to cause no damage to persons or property, one may wonder whether those actions involve any force at all and are relevant in analysing the distinction between the concepts of use of force and police measures. In fact, the book does not contain any specific analysis of the notion of armed force that seems common to both concepts.

Corten also ascribes a significant role to the intention of states. At first glance, this may be quite unexpected, especially given his restrictive approach adopted in the matter. It is indeed well known that intention is a very subjective and, therefore, malleable criterion, which may be easily interpreted by states in a way as supporting their views. It is, moreover, difficult to identify it in practice. Corten is perfectly aware of such risks and makes an effort to define this criterion in a restrictive way. While asserting that such a criterion merely means the intention to coerce one state to do or not to do something by force,Footnote ⁵³ he immediately makes it clear that it must not be confused with the objectives or motives pursued by a state using force. Basically, Corten wants to show that his definition of intention is unrelated to the argument – which is sometimes invoked in legal literature, as will be explained belowFootnote ⁵⁴ – that a military action should not be considered as amounting to a use of force prohibited under Article 2(4) of the UN Charter if it is not directed against the territorial integrity or political independence of a state or if it is exercised in a manner compatible with UN purposes. In Corten's view, this argument bears on the scope of the prohibition on the use of force while the intention criterion, as he defines it, bears on the qualification to be given to a coercive action, either a use of force or a police measure.Footnote ⁵⁵ This conceptual distinction is clearly relevant. Yet, Corten's definition of the intention criterion remains flexible. One may still hardly apply it in practice. It is, moreover, difficult to differentiate it from the criterion based on the objectives of a use of force. One could perfectly argue that, when invading Panama in 1989, the United States did not want to coerce Panama to do or not to do something or, more generally, to act against this state, but had the mere intention, for example, to protect their citizens. Why, when applying the intention criterion alone, could one not qualify this use of force by the United States as not amounting to a use of force within the meaning of Article 2(4) of the UN Charter? Why should one accept the validity of such argument only in cases like the Israeli abduction of Adolf Eichmann in Argentina? More generally speaking, all the limited self-defence actions undertaken in response to attacks by non-state actors could clearly be argued as not having involved any intention to coerce the state in the territory of which they took place to do or not to do something. Yet, they were considered by the intervening state as well as other states as an international use of force whose justification was claimed to be based on self-defence under Article 51 of the UN Charter.Footnote ⁵⁶

Such problems could be attenuated by restricting the meaning of the notion of intention and by not placing it on an equal footing with the gravity criterion, but rather by subordinating the former to the latter. State practiceFootnote ⁵⁷ clearly evidences that intention is an element taking part in the qualification of an armed action as a use of force or, at least, as an armed attack. However, this element must be related not to the content (or object) of the intention – that is, requiring, as Corten contends it, an assessment of whether the intervening state intended to coerce another state to do or not to do something – but rather to the existence of the intention itself – that is, requiring an assessment of whether the armed force was deliberate and not accidental or erroneously resorted to. Accordingly, frontier incidents should not be considered as uses of force within the meaning of Article 2(4) of the UN Charter, since such incidents often result from the crossing by mistake of frontiers by military troops or the uses of force by soldiers who had not been given any instruction from their government to that end.Footnote ⁵⁸ Although more restrictive, such conception of the notion of intention still remains subjective and difficult to assess in practice. This shows that intention must be ‘subordinated’ to the gravity criterion. This criterion, which entails a more objective assessment, actually proves to be an easy means for revealing prima facie the existence or not of the intention, a high level of gravity of force supposing that such a force was not accidentally or erroneously resorted to, while a low level of gravity suggests the absence of any intention. This role of presumption, which is played by the gravity criterion with regard to intention, may find some support in state practiceFootnote ⁵⁹ and legal literature.Footnote ⁶⁰ It seems better suited to guarantee international legal security, as it gives priority to more objective factors without preventing at the same time adapting the solution to the particular circumstances of the case. Indeed, as a presumption, the conclusion reached on the basis of the gravity criterion may be rebutted by the proof of the existence or absence of intention.

3.2. Threat of force

Having analysed the notion of use of force, Corten undertakes the study of the second situation, which is prohibited under Article 2(4) of the UN Charter – that is, threat of force. This study is particularly welcome, since there are very few general reflections on that issue in legal literature.Footnote ⁶¹ He analyses the meaning of the notion of threat of force as well as the legal regime applicable to this situation. Based on a detailed analysis of state practice, international case law, and the work of the International Law Commission (ILC), Corten's definition of the threat of force is clearly restrictive. In Corten's view, only identified, clearly established, and expressly formulated threats fall under Article 2(4) of the UN Charter. In other words, vague risks and uncertain and implied threats are excluded from the scope of this provision. Corten's positions about the scope of the prohibition on the threat of force are almost entirely devoted to refuting the arguments put forward by Romana Sadurska on this issue in a paper published in 1988 in the American Journal of International Law.Footnote ⁶² In her paper, Romana Sadurska concludes that there is a ‘disjunction between the use of force as such, which is strictly prohibited by Article (4), and the threat of such use of force, which is supposedly prohibited only in a more flexible manner’.Footnote ⁶³ Defending the symmetry between the threat of force and the corresponding use of force, Corten convincingly deconstructs all the arguments allegedly supporting the opposite view. Again, such a deconstruction is based on an all-encompassing study of state practice and international case law.Footnote ⁶⁴

4.1. Do the prohibitions of the use of force and self-defence apply to non-state actors?

This chapter on the application of the law on the use of force to non-state actors is divided into two parts, the first being devoted to non-state political entities and the second to private groups. In the first part, Corten starts by demonstrating the inapplicability of the rule prohibiting the use of force to civil wars, mainly by relying on the traditional positions usually upheld by states in that respectFootnote ⁶⁶ and by convincingly countering the idea supported by some scholars that changes have been made to that matter by recent state practice, such as the civil war in Yugoslavia in the 1990s.Footnote ⁶⁷ He then draws a similar conclusion regarding the applicability of the law on the use of force to both national liberation strugglesFootnote ⁶⁸ and entities of controversial status.Footnote ⁶⁹ This conclusion is mainly founded upon the analysis of declarations made by states during the preparatory works of the general UNGA resolutions on use of force as well as in relation to some particular precedents. This analysis reveals a clear disagreement among different groups of states about these issues. The socialist and non-aligned states were in favour of applying the law on the use of force to national liberation struggles, contrary to the Western states, whereas the latter were in favour of recognizing the applicability of this law to entities of controversial status, unlike the socialist and non-aligned states.Footnote ⁷⁰ This disagreement could have led the author to acknowledge that ambiguity remains on that point. Yet, Corten's unequivocal rejection of the applicability of the law on the use of force, including self-defence, to the aforesaid entities in the light of state practice may be explained by both his assumption that it was initially the case and his conclusion that no general consensus has emerged in practice evidencing some change in that respect. This is actually in line with the general questionable methodological choice that he has implicitly endorsed, according to which, as mentioned above, he starts his analysis of contemporary state practice by presuming that all the currently debated applications of the law on the use of force were originally, directly or indirectly, prohibited (cf. sub-subsection 2.2.2, supra).

In any event, the inapplicability of the prohibition on the use of force with respect to entities of controversial status may now find clear and direct support in ICJ case law, especially since the ICJ 2010 advisory opinion in the Kosovo case. The Court indeed held in this Opinion:

It is true that the entity at stake was an entity of controversial status, namely Kosovo. However, nothing a priori precludes one from applying the ICJ conclusions to any non-state entity. Those conclusions seem to contradict the position upheld by the Independent International Fact-Finding Mission on the conflict in Georgia – convincingly criticized by CortenFootnote ⁷² – that ‘in so far as the parties to the internal armed conflict accept . . . them in particular agreements, Articles 2(4) and 51 of the UN Charter . . . apply to their relations'.Footnote ⁷³

Although particularly well structured and illustrated, Corten's reasoning is sometimes confusing. In some parts of his argumentation, he indeed analyses the question of the applicability of the law on the use of force to non-state actors by wondering whether it has been admitted that those actors could be the authors of an armed attack triggering the right of self-defence of the victim state.Footnote ⁷⁴ Yet, the law with respect to which the applicability to non-state actors is questioned encompasses the prohibition on the use of force and the right of self-defence. The issue at stake is, therefore, to analyse whether those actors are bound by the prohibition on the use of force under Article 2(4) of the UN Charter and/or are holders of the right to respond in self-defence to an armed attack under Article 51 of this Charter and not whether they may commit an armed attack. If it was considered that attacks by non-state actors might amount to an armed attack in the sense of Article 51, then this would not, however, mean that the law on the use of force would be applicable to them.

This remark is particularly helpful in responding to the theoretical objections made by Corten against the development of the law on the use of force towards the recognition of a right to self-defence in response to an armed attack committed by non-state actors only – that is, attacks by private groups, especially terrorists. Contrary to the mainstream of the legal literature,Footnote ⁷⁵ Corten concludes in the second part of this chapter that such a right has not emerged yet. His reasoning is formulated in four main steps. The first relates to the letter as well as the object and purpose of the law on the use of force.Footnote ⁷⁶ Corten's main argument in this regard is that state declarations clearly evidence that the prohibition on the use of force and the right of self-defence are still deemed as concerning only ‘international relations' as relations among states.Footnote ⁷⁷ The author moreover contends that the existing legal framework provides sufficient means for fighting against terrorism.Footnote ⁷⁸ Finally, he claims that the recognition of a right to self-defence in response to private armed attacks would lead to absurd legal consequences, as, in his view, private groups should be considered as bound by Article 2(4) of the UN Charter and, having the right to act in self-defence in case they are themselves attacked, they should be recognized as having an international legal personality.Footnote ⁷⁹ The second, third, and fourth elements upon which Corten bases his position against a right to self-defence in response to private armed attacks are state practice,Footnote ⁸⁰ the work of the ILC,Footnote ⁸¹ and ICJ case law,Footnote ⁸² respectively. Those arguments deserve to be discussed.

A first general comment is that the question of the right to self-defence in response to private armed attacks does not seem to have been addressed from the most suitable perspective. Indeed, this question does not appear to be related to the applicability of the law on the use of force to private groups. Those groups may perfectly well be the authors of a factual occurrence, an armed attack – which means a force of some level of gravity (cf. subsection 3.1, supra) – without being bound by the prohibition on the use of force or being the holders of a right of self-defence under the UN Charter – that is, without the regulation on inter-state use of force being applicable to them. To be the author of an armed attack triggering the right of self-defence and to be the holder of such a right are definitely not the same thing. It seems difficult to share Corten's implicit reasoning that private groups must logically be considered as having themselves a right to self-defence when they are attacked – and, therefore, as enjoying an international legal personality – once it is admitted that they can commit an armed attack under Article 51 of the UN Charter. Although a dog can attack a person and possibly trigger this person's right of self-defence under the national legislation of some states,Footnote ⁸³ it is clear that this dog has no right of self-defence if it is itself attacked – nor does it enjoy any legal personality. In any event, one could expect that more specific developments would have been provided for by the author in order to support this highly questionable position that he indirectly endorses.

Moreover, admitting that private groups may commit an armed attack triggering the right of self-defence of the victim state does not mean that the exercise of this right would no longer be concerned with international relations ‘as relations among states’. Again, it is uncontested that the right of self-defence, like the prohibition on the use of force,Footnote ⁸⁴ is confined to the sphere of relations between states. Yet, even if exercised in response to a private armed attack, the action undertaken in self-defence by the victim state will remain in the sphere of international relations when it is exercised in the territory of another state – which must, moreover, as will be seen below, not be controlled by the state acting in self-defence. In this case, as in any other cases in which ‘classical self-defence’ is exercised, the action is still carried out by a state, which is the holder of the right of self-defence under Article 51 of the UN Charter, in the territory of another state, whose protection against unlawful use of force under Article 2(4) of the UN Charter should have been considered as violated if this action had not been justified by self-defence. It is therefore clear that such action in self-defence, although responding to a private armed attack, is still concerned with relations among states.

In fact, there is only one particular issue at stake that relates to the applicability of the law: indentifying the international rule under which the attacks committed by the non-state actors can be considered as unlawful and, as a result of such unlawfulness, trigger the right of self-defence of the victim state. Indeed, in order to amount to an armed attack within the meaning of the law on self-defence, the attacks must consist of a force that is not only serious and deliberate (cf. subsection 3.1, supra), but also unlawful. Again, it is clear that such unlawfulness cannot be assessed in the light of Article 2(4) of the UN Charter with respect to private armed attacks, since this article only applies to states. Referring to this rule is only relevant when determining whether an attack committed by a state is unlawful and may trigger the right of self-defence of the victim state. Yet, state practice clearly evidences that attacks committed by non-state actors have sometimes been clearly and unanimously judged as unlawful.Footnote ⁸⁵ It may be argued that the rule in the light of which this judgement can be made is the prohibition on international terrorism. Although the comprehensive convention prohibiting international terrorism has not been adopted yet, debates held regarding the current draft clearly show that the adoption thereof is delayed because of problems related to the scope of the prohibition, mainly the issue of excluding some forms of struggle by non-state actors, such as the fight for self-determination,Footnote ⁸⁶ from the definition of terrorism. The principle itself of prohibiting international terrorism is not contested.Footnote ⁸⁷ In each particular case, states can then assess whether attacks committed by non-state actors are unlawful acts of international terrorism without basing such a judgement on Article 2(4) of the UN Charter. In the past, some states, mainly the socialist ones, have expressly claimed that the attacks committed by non-state actors could not trigger the right of self-defence of the alleged victim state, since those attacks were not unlawful terrorist acts, but legitimate acts of resistance against occupying powers or colonial regimes.Footnote ⁸⁸

Another general comment on Corten's reasoning regarding the right to respond in self-defence to private armed attacks concerns his interpretation of state practice on that issue. The author, indeed, concludes that the practice does not evidence any change in the law on the use of force towards the recognition of such a right. This conclusion is based upon two particular interpretations. First, Corten rejects a number of precedents as being not relevant for discussing the issue at stake. In his view, some precedents, such as the US Operation Enduring Freedom in Afghanistan in response to the 9/11 attacksFootnote ⁸⁹ or the Israeli invasion of Lebanon in 2006 in reaction to the rocket attacks by Hezbollah,Footnote ⁹⁰ are precedents in which the action in self-defence was considered as responding not only to the attacks by the non-state actors, but also to the armed attack committed by the host state, resulting from the implication of this state in these attacks. According to Corten, those precedents are only pertinent in analysing whether the traditional definition of state armed attack, which includes attacks committed by armed bands in which a state is substantially involved, has evolved and now encompasses minor state involvements in attacks by private groups. As a result, the author only discusses those precedents in relation to that question (cf. subsection 4.5, infra). In a second step, Corten nonetheless acknowledges the existence of some precedents in which self-defence was apparently exercised in response to attacks committed by non-state actors only, such as the 2008 Turkish intervention in Iraq in response to the Kurdish attacks from the Iraqi territory.Footnote ⁹¹ After analysing those precedents, he concludes, however, that they did not lead to any development of the law on the use of force, since the claimed change was not clearly formulated nor was it accepted by the international community as a whole.

One must admit that the latter precedents are not clear and sufficiently repeated to enable one to formulate any firm conclusion on the issue at stake. However, some of the precedents rejected by Corten carry more weight, since the right of self-defence was clearly invoked and accepted by a significant number of states,Footnote ⁹² at least as a matter of principle.Footnote ⁹³ Yet, there is actually no decisive reason to consider those precedents as not relevant. It is true that, in each of those precedents, the intervening state and some other states have emphasized the existence of a link between the attacks committed by the non-state actors and the host state, the latter being generally considered as unwilling or unable to put an end to the attacks committed from its territory. However, the attacks that have been considered as the ‘armed attack’ triggering the right of self-defence under Article 51 of the UN Charter were those committed by the non-state actors and not the low level of implication of the host state in these attacks. While many states expressly qualified the attacks by the non-state actors as an armed attack or asserted that the state acting in self-defence had been attacked by the non-state actors,Footnote ⁹⁴ they never qualified the host state's behaviour in the same way nor considered that the state acting in self-defence had been (indirectly) attacked by this state.Footnote ⁹⁵ As argued in more detail elsewhere,Footnote ⁹⁶ such references to the host state's behaviour do not actually play any role in the definition of the attacks as armed attacks in the meaning of Article 51 of the UN Charter. They merely serve as a means for emphasizing the necessity of the action in self-defence. Indeed, the condition of necessity requires, in one of its multiple aspects, that the action in self-defence be resorted to as a last resort – that is, after all the practical alternatives to self-defence have been exhausted.Footnote ⁹⁷ The fact that the host state is considered as unable or unwilling to bring to book the activities of the groups that are hostile to another state is evidence that the latter can no longer rely on any action from the host state in order to defend itself, which increases the necessity to act in self-defence. Corten's particular interpretation of the aforementioned precedents, according to which emphasis put on the host state's behaviour means that this state was accused of an armed attack, actually seems to result from his intimate – but, as seen above, highly questionable – conviction that the right of self-defence will lose its international character when exercised in response to private armed attacks and the ensuing tendency to seek as much as possible to attribute such attacks to a state. Those precedents should not, therefore, be disregarded.

As a result, a clear trend towards recognizing a right to self-defence in response to non-state actors may be inferred from such precedents altogether with the other less-clear precedents. This conclusion is particularly arguable if one keeps in mind that, contrary to Corten's implicit methodological approach, the claimed evolution only implies an interpretation rather than the modification of a former rule and must not accordingly meet the strict conditions regarding the quality and quantity of state practice under which a modification of the law on the use of force is permissible (cf. sub-subsection 2.2.2, supra). Indeed, the most obvious unsettled question regarding this law at the time of the drafting of the UN Charter was the right to respond in self-defence to private armed attacks. Although this right was not expressly admitted at the San Francisco Conference, it was not expressly excluded either, whether before or at the time of the drafting of the Charter. Article 51 of this Charter does not expressly require that the armed attack triggering the right of self-defence has to be committed by a state. Even if it may be argued that the law of self-defence has been implicitly interpreted as entailing such a requirement,Footnote ⁹⁸ this arguable position nevertheless remains a matter of interpretation. It does not assert the existence of a clearly well-established rule whose evolution should be considered as a modification thereof.

In a last part of the chapter on the applicability of the law on the use of force to non-state entities, Corten refers to ICJ case law in order to support his conclusion against the existence of a right to self-defence in response to private armed attacks. Yet, as already mentioned above,Footnote ⁹⁹ the Court never expressly ruled out such a right. It was actually never asked to pronounce on this issue. In both the Nicaragua and Armed Activities cases, the United States and Uganda accused Nicaragua and Congo of having committed an armed attack by themselves – that is, through their involvement in the attacks launched in Salvador and Uganda by the anti-Salvadorean and anti-Ugandan rebels, respectively. They never claimed to have acted in self-defence, only in response to the attacks committed by those rebels. Nothing may therefore be inferred from those cases on the right to use force in self-defence in response to private armed attacks. It is nonetheless true that the Court stated in the Wall opinion that ‘Article 51 of the Charter recognizes the existence of an inherent right of self-defence in the case of armed attack by one State against another State’.Footnote ¹⁰⁰ Yet, this reference is particularly brief. It must actually be read in the context of the whole of the Court's reasoning on this issue, especially in light of the subsequent sentences:

The Court's reference to an ‘armed attack by one State’ may arguably be interpreted as meaning that the law of self-defence is not relevant when the armed attack is not committed by a foreign state or does come from a foreign territory that is controlled by the state acting in self-defence. Such a conclusion is consistent, since invoking the law of self-defence is only relevant if it serves as a means for justifying a use of force that would otherwise be contrary to the prohibition on the use of force. This necessarily entails that such use of force has affected the sovereignty of another state and has not been exercised in a territory controlled by the state acting in self-defence. In the latter case, the use of force should normally be assessed in the light of the jus in bello rules on occupation.Footnote ¹⁰² In sum, those considerations do not preclude the assertion of a right to self-defence in response to private armed attacks when those attacks are committed from a foreign territory, which is not moreover controlled by the state acting in self-defence.

4.2. Can circumstances precluding unlawfulness be invoked to justify a use of force?

A specific analysis of the possibility to invoke circumstances precluding wrongfulness in the field of use of force was particularly needed. Several scholars, indeed, argue that such circumstances, especially the state of necessity, can justify some limited military actions, mainly interventions in a foreign state for humanitarian purposesFootnote ¹⁰³ or for targeting terrorist groups sheltered by another state.Footnote ¹⁰⁴ Such a position is generally derived from the idea that only one part of the prohibition on the use of force, namely the prohibition on aggression, has a peremptory nature. As a result, Article 26 of the ILC's articles of state responsibility, according to which no circumstance may ‘preclud[e] the wrongfulness of any act of a State which is not in conformity with an obligation arising under a peremptory norm’, would not prevent the invocation of a circumstance precluding wrongfulness to justify a minor use of force that would not violate the prohibition on aggression. Such reasoning was suggested by the ILC's Special Rapporteur on state responsibility.Footnote ¹⁰⁵

Corten convincingly refutes all these arguments and reasoning in three main steps. First, he demonstrates that the whole prohibition on the use of force has a peremptory nature. Such demonstration is carried out with great care. The author mentions an impressive list of declarations in which states expressly consider the prohibition on the use of force as pertaining to jus cogens.Footnote ¹⁰⁶ He also analyses the treaty practice from which he infers that no derogation from the prohibition on the use of force has ever been foreseen by the state parties to the treaties dealing with the use of force.Footnote ¹⁰⁷ Second, Corten demonstrates that the ILC has not finally endorsed the opinion, evoked earlier in the context of its work on state responsibility, that a circumstance precluding wrongfulness could justify a minor use of force. This demonstration is particularly convincing.Footnote ¹⁰⁸ As emphasized by the author, the ILC finally stated that it did not want to settle this issue,Footnote ¹⁰⁹ while the new Special Rapporteur on state responsibility explicitly asserted his reluctance to divide the prohibition on the use of force and attribute a peremptory nature only to one part of it.Footnote ¹¹⁰ This reluctance also seems to have been shared by the ILC itself, as confirmed in other parts of its work.Footnote ¹¹¹ Third, and finally, Corten demonstrates that the state of necessity has never been accepted in state practice as a legal basis for justifying any use of force. Many precedents are scrutinized in that respect.Footnote ¹¹² The author persuasively shows that, in many of those precedents, the state of necessity was not the true official legal justification invoked by the intervening state, whereas it was not elaborately articulated in the rare precedents in which it seems to have been called upon.

Although Corten's conclusions are incontestable, parts of his reasoning call for some comments. A first comment concerns the list of declarations that he mentions in order to support his conclusion that the whole prohibition on the use of force is considered by the entire international community as having a peremptory nature. However, apart from a few states,Footnote ¹¹³ the majority of states did not expressly claim that both the prohibition on the use of force and the prohibition on aggression pertained to jus cogens. They merely recognized that the prohibition on the use of force had such a nature, without admittedly limiting such a nature to the prohibition on aggression but without specifying either that this nature covered the whole prohibition, although the issue had already been raised within the ILC. Therefore, it is not sure that, when having made such declarations, those states were aware of or felt concerned by this issue. One may then wonder whether a clear and firm opinio juris may be inferred from those declarations against the attribution of a peremptory nature only to one part of the prohibition. Another and more fundamental argument, which could have been put forward against such attribution, is to contest the distinction between the notions of use of force and aggression based on the gravity of the force resorted to. It is difficult to imagine any use of force, which already entails a force of some gravity – otherwise it should be analysed as a police measure – (cf. subsection 3.1, supra), that would unlawfully be used against a state without amounting to an aggression. The question is not discussed by Corten in this chapter. It is only briefly addressed in the chapter on self-defence (cf. subsection 4.5, infra). As will be seen below, Corten asserts that an armed attack within the meaning of Article 51 of the UN Charter must be of a higher level of gravity than a use of force. Yet, the issue is not discussed in detail.

Another comment on Corten's reasoning concerns his criticisms vis-à-vis the position sustained by Judge Simma in his separate opinion in the Oil Platforms case, according to which a limited unlawful use of force, short of an armed attack, could justify in response a defensive limited use of force, short of an action in self-defence.Footnote ¹¹⁴ This position has the merit of allowing states victim of a minor use of force to respond to it militarily in a proportionate manner without altering the traditional idea that only a serious use of force may amount to an armed attack triggering the right of self-defence. Contrary to what is argued by Corten,Footnote ¹¹⁵ Judge Simma's argument does not seem to relate to the notion of armed reprisals as those reprisals are classically understood and now unanimously prohibited. Unlike the military measure described by Judge Simma, armed reprisals do not pursue a defensive objective, but more generally aim at compelling a state by force to comply with some of its international obligations. The element triggering the armed reprisals can, therefore, consist of the violation of any obligation binding upon the state against which reprisals are exercised and not only of the violation of the prohibition on the use of force. Besides, Corten calls into question the interpretation by Judge Simma of the statements made by the ICJ in the Nicaragua case and upon which the judge has defended his position.Footnote ¹¹⁶ Yet, the abundant literature on the subject clearly shows that the aforementioned ICJ statements are particularly ambiguous and clearly leave the door open to the interpretation sustained by Judge Simma.Footnote ¹¹⁷ Finally, Corten notes that, in the Oil Platforms case, the ICJ did not discuss whether the United States, although not the victim of a serious use of force amounting to an armed attack by Iran, could have responded in a limited way to this use of force, in accordance with Judge Simma's idea. In Corten's view, such a silence is indicative of the reluctance of the Court to endorse this idea.Footnote ¹¹⁸ Yet, the ICJ's silence is entirely consistent. Indeed, Judge Simma's argument is only applicable if the use of force that does not amount to an armed attack is considered as such because of its low level of material gravity. It is only against such a use of force that one can imagine a response that is also of limited gravity and, more particularly, of a lower level of material gravity than the one characterizing any action in self-defence. In the Oil Platforms case, it is true that the Court did not qualify the alleged Iranian use of force as sufficiently grave to amount to an armed attack against the United States. However, the gravity considered by the Court was related not to the material gravity of the contemplated Iranian use of force, but to the intention of Iran to specifically target the United States.Footnote ¹¹⁹ Yet, such intentional gravity, unlike material gravity, cannot be graduated into different levels. One can hardly envisage the state victim of an indiscriminate use of force resorting to a limited ‘intentional’ military response. The intention to target a specific state does or does not exist. As a result, Judge Simma's argument could not be applied in that case and nothing should be inferred from the ICJ's silence in that respect.

4.3. Intervention by invitation

Intervention by invitation is another issue that is neglected in the legal literature on the use of force. Yet, this issue raises many legal questions. Corten addresses all these questions in this chapter. The first and fundamental question relates to the compatibility of consented militarily interventions with the prohibition on the use of force. The author rightly questions in this respect the conclusion reached by the ILC in its final draft on state responsibility, that consent, as a circumstance precluding wrongfulness, can justify a use of force. As he emphasizes, this conclusion is quite paradoxical, since no such circumstance may preclude the wrongfulness resulting from the violation of a peremptory norm like the prohibition on the use of force.Footnote ¹²⁰ He rightly contends that, in relation to use of force, consent does actually not act as a secondary rule (i.e., as a circumstance precluding wrongfulness), but as an intrinsic element of a primary rule, entailing that the consented intervention is actually outside the scope of the prohibition on the use of force. He nonetheless acknowledges that consenting to a genuine use of force could hypothetically be contrary to the prohibition on the use of force if consent is provided in a general treaty and not merely given on an ad hoc basis. However, according to him, no consented military operation has ever been conducted in practice on the basis of such kind of treaty.Footnote ¹²¹

Corten then analyses the condition under which consent is validly given.Footnote ¹²² On the basis of an in-depth scrutiny of state practice, he concludes that consent must be given prior to the intervention by the state's highest authorities, which excludes the opposition and subaltern authorities. Consent must also be unvitiated, mainly not being given under coercion, as well as clearly established and relevant – that is, be unambiguous, even if it does not have to be given within specific formalities, and bear on the acts that it seeks to justify.

Corten finally addresses the specific problems of the intervention by invitation in the context of internal armed conflicts, such situations being highly problematic notably because several authorities may claim to be the legitimate government. The question of the authority to which the invitation can be consented to is first discussed.Footnote ¹²³ Corten concludes that the consent can only come from the government that is internationally recognized as well as exercising an effective authority over the territory. Then, he addresses the most debated question pertaining to the matter, whether a third state is allowed to intervene in civil wars at the invitation of the internationally recognized and effective government.Footnote ¹²⁴ In this respect, reference is usually made in legal literature to the opposing views upheld by the ICJ in the Nicaragua case and the International Law Institute in its resolution adopted in 1975. The ICJ a priori recognizes the lawfulness of the intervention at the request of the governmentFootnote ¹²⁵ while the Institute rejects it if it does not respond to a prior outside intervention,Footnote ¹²⁶ mainly because of the requirement to respect the right of peoples to self-determination, as contended by several members of the Institute.Footnote ¹²⁷ Corten's position is clearly in line with the resolution adopted by the Institute. It is nonetheless more explicit and comprehensive. Analysing this issue from the angle of the purpose of the intervention by invitation, Corten clearly concludes that interventions whose purpose is to influence the outcome of the conflict in violation of the right of people to self-determination are unlawful in principle, while confirming the lawfulness of interventions responding to a prior outside intervention. However, he also expressly recognizes the lawfulness of other interventions by invitation – those whose purpose is precisely not to impact upon the conflict, but to carry out humanitarian aid or to maintain law and order or peace.Footnote ¹²⁸

These last considerations merit several remarks. One may first reflect on their relevance. Indeed, the aforementioned interventions, contrary to those whose purpose is to impact upon the conflict or to respond to an outside interference, do not seem to entail any genuine use of force and, as a result, do not seem to be pertinent in any debate on the lawfulness of foreign military interventions in civil wars or, more generally, in any discussion on the use of force. Those considerations nevertheless help to get a more comprehensive view on the question of intervention by invitation. They actually appear as a decisive argument enabling Corten to argue for the unlawfulness of foreign interventions whose design is incompatible with the right of self-determination. Indeed, given the limited state practice directly evidencing the unlawfulness of such controversial interventions, Corten goes to the trouble of showing that the only interventions that have been authorized are those designed to deliver humanitarian aid or to maintain law and order or peace as well as those undertaken in response to a prior outside interference, assuming that the other types of intervention are prohibited under international law.

This reasoning is nonetheless puzzling. It is not because some interventions are authorized that other kinds of intervention are necessarily prohibited. The purpose of such reasoning actually seems to be to emphasize the lack of any practice supporting the controversial interventions. As already shown above, this is a typical debatable aspect of Corten's method, which consists of analysing state practice from the presumed starting point that there is a prohibition with regard to the use-of-force aspect under review (cf. sub-subsection 2.2.2, supra). Corten is then able to demonstrate the maintaining of this presumed prohibition by showing that there are no or only few practices evidencing the modification thereof. This chapter is nonetheless specific in the sense that Corten expressly recognizes the lawfulness of some particular kinds of contemporary military intervention. If one follows the restrictive positivist method that he has adopted, such recognition should logically be based on a particularly clear, widespread, and repeated state practice. Yet, the state practice to which he refers lacks such features, especially with regard to the establishment of the right to intervene for humanitarian purposes or for maintaining law and order. The lawfulness of such interventions is indeed based on few as well as merely suggestive state practices.Footnote ¹²⁹ This contrasts with the abundant state practice mentionedFootnote ¹³⁰ in order to support the lawfulness of the intervention designed to respond to a prior foreign intervention.Footnote ¹³¹ One may wonder whether such a difference in treatment is justified by the fact that the right to intervene for humanitarian purposes or for maintaining law and order does not involve a genuine use of force and that, accordingly, Corten's restrictive method does not apply to this issue. The author does not expressly defend such a view. One is therefore left a little confused with respect to the method that has been adopted in this chapter.

4.4. Intervention authorized by the UN Security Council

In contrast with the intervention by invitation, the issue of intervention authorized by the UN Security Council has been widely covered by the legal literature on the use of force.Footnote ¹³² Corten has divided the analysis of this issue into two parts, the first being devoted to the general legal regime of interventions authorized by the UN Security Council and the second to the more controversial question of presumed authorizations. In the first part, Corten starts by confirming the now well-established lawfulness of armed interventions authorized by the UN Security Council,Footnote ¹³³ before addressing the two general conditions under which such interventions are lawful:Footnote ¹³⁴ first, the conformity of the resolution authorizing the intervention with the UN Charter (whereby Corten attributes a wide margin of discretion to the UN Security Council, apart from the obligation to qualify the situation as a threat to peace, a breach of the peace or an act of aggression); and, second, the conformity of the military intervention with the UNSC resolution (whereby the question is raised of whether the expression ‘all necessary means or measures’ mentioned in the UNSC resolution entails the right to use of force and whether the intervention has been limited to the circumstances provided in the resolution). Corten ends this first part by persuasively demonstrating that military interventions cannot be lawfully ‘authorized’ by another UN body or another subject of international law – that is, by either the UN General Assembly or a regional organization.Footnote ¹³⁵

Apart from questionable clear-cut conclusions regarding the interpretation of some African texts or declarations on that matter, the meaning of which should be considered as inconclusive, since they have not been put into practice yet (cf. sub-subsection 2.2.2, supra), Corten's considerations are particularly convincing. The same may be said about the opinions that he elaborates concerning the more debated question of presumed authorizations. He rightly refutes any interpretation inferring the lawfulness of presumed authorizations from state practice.Footnote ¹³⁶ The main argument is that, in the majority of precedents, this contentious legal basis was never clearly invoked by the intervening states, the justification of the intervention being usually based on another right, such as self-defence or exceptional right to human intervention. Moreover, in all the precedents, including the few in which presumed authorization was invoked, especially by the United States with respect to their interventions in Iraq in the 1990s and in 2003, this legal basis has never been accepted by all the other states. The unlawfulness of presumed authorization is evidenced not only by the lack of precedents supporting it, but also by the declarations, quoted by Corten, in which many states expressly emphasized their reluctance to admit such a legal basis,Footnote ¹³⁷ as well as by the incompatibility of the latter with the UN Charter.Footnote ¹³⁸

4.5. Self-defence

Self-defence is the oldest and most debated institution under which states are allowed to resort to force.Footnote ¹³⁹ It was already referred to by states before the now well-established general prohibition on the use of force,Footnote ¹⁴⁰ although its contemporary scope and meaning were shaped at the time at which this prohibition was emerging, during the interwar period. It is understandable that, being the only legal means for unilaterally resorting to force, it was abundantly – most often unlawfully – invoked by states, especially during the Cold War period, when no collective use of force through the UN Security Council was available. Current state practice raises several controversial questions with respect to self-defence. Many of them are related to the scope of the notion of armed attack, whose occurrence, according to Article 51 of the UN Charter, authorizes the triggering of the right of self-defence. Corten analyses those questions in a first part before addressing in a second part the conditions under which this right must be exercised – that is, the necessity and proportionality of the response in self-defence as well as the relationships between this response and the measures decided by the UN Security Council.

In the first part, Corten mainly addresses two controversial issues: preventive self-defence and self-defence in response to indirect aggression. Before analysing these issues, he lists a series of positions that he considers as ‘hardly contestable’.Footnote ¹⁴¹ He assumes, for example, that an armed attack is a use of force of certain gravity.Footnote ¹⁴² It is, however, doubtful that such a position is unanimously accepted. This is evidenced by the current doctrinal debates on the right to respond by force to a use of force of minor gravity. Such a right has been very much discussed, notably in the framework of the resolution on self-defence adopted in 2007 by the International Law InstituteFootnote ¹⁴³ and concerning the Israeli intervention in Gaza in 2009.Footnote ¹⁴⁴ This question is actually an old doctrinal question and there are strong arguments for contending that, provided that it is directed against a state and is unlawful, any use of force may amount to an armed attack triggering a proportionate response in self-defence.Footnote ¹⁴⁵

Yet, it is certainly not currently the most debated question. One of such questions is the right of preventive self-defence, namely the right to counter the threat of an armed attack in self-defence. Corten's main argument is that such a right does not exist in the current state of international law, since no general consensus in favour of it has emerged from state practice. While some states are reluctant to prohibit preventive self-defence in general terms, a significant number of states have always been opposed to recognizing it.Footnote ¹⁴⁶ This is an indisputable fact. However, Corten's argument is again based on the questionable methodological approach, according to which, as already mentioned above (cf. sub-subsection 2.2.2, supra), state practice is analysed from the presumption that all the currently debated applications of the law on the use of force, including preventive self-defence, were originally clearly prohibited. Yet, contrary to Corten's view, the existence of a clear-cut prohibition on preventive self-defence at the time of the drafting of the UN Charter is far from being obvious. The status of this right was controversial until the end of the Second World War. It had not been expressly admitted by states at the end of the interwar period, but it had not been excluded either, although it was invoked several times in practice.Footnote ¹⁴⁷ Moreover, it seems to have been recognized in some treaties adopted just before the outbreak of the Second World War.Footnote ¹⁴⁸ It is well known that the scope of the law of self-defence, especially the notion of armed attack, was not subject to any general discussion during the conferences preceding the adoption of the UN Charter and that there was no intention to bar the application of customary law pre-dating the UN Charter on that issue.Footnote ¹⁴⁹ Although, as mentioned by Corten, some states nonetheless expressly declared that the right of self-defence could only be resorted to after an armed attack has occurred,Footnote ¹⁵⁰ such declarations are not enough – particularly if one follows Corten's method – to establish a clear and definite interpretation of Article 51 of the UN Charter, the past and subsequent practice of those states moreover evidencing a contrary position. As there was originally no well-established prohibition of the right of preventive self-defence, contrary to Corten's assumption, the manifest disagreement emerging from contemporary state practice with respect to this issue should lead us to the modest conclusion that the ambiguous status of preventive self-defence has not been clarified yet. As already mentioned above, ICJ case law is not helpful in that respect, since the Court has never been asked and has never ruled on the question of preventive self-defence (cf. sub-subsection 2.2.2, supra). The most that could be said about this question is that preventive self-defence is (only) presumably unlawful. This may be inferred both from the attitude of states, which, although opposed to prohibiting it in general terms, are nevertheless reluctant to invoke it,Footnote ¹⁵¹ and from the disapprobation of the many preventive non-authorized uses of force, although the latter have not been condemned only on the basis of the preventive nature of the force.Footnote ¹⁵²

There are also some doubts with respect to the right to prevent an imminent threat of armed attack in self-defence, namely the right of ‘anticipatory self-defence’ (cf. sub-subsection 2.2.2, supra). Corten rejects any form of preventive self-defence, including anticipatory self-defence. He nonetheless agrees with the right of a state to act in self-defence in order to counter a use of force, which, although not having reached its territory, ‘has . . . materially begun’.Footnote ¹⁵³ This is close to what some scholars qualify, according to Dinstein's wording,Footnote ¹⁵⁴ as ‘interceptive self-defence’.Footnote ¹⁵⁵ In Corten's view, such self-defence has no preventive nature, since it is responding to an armed attack. Yet, this position is debatable, for two main reasons. First, it raises a problem of legal justification. This position is indeed based on a particular definition of the notion of armed attack, which is normally viewed as consisting of an actual violation of the territorial sovereignty of a foreign state by force (or, arguably, in actual attacks against the agents or nationals of a foreign state). Moreover, such a position is not supported by any reference to state practice or international case law. Being a priori legitimate, this right to respond in self-defence to an already materialized use of force should preferably be founded on an alternative legal basis, taking into account state practice. It is true, as argued by Corten, that the right of anticipatory self-defence has not been explicitly agreed upon by all states.Footnote ¹⁵⁶ Yet, the limited opposition to such a right, when it has not been manifestly formulated only for political purposes,Footnote ¹⁵⁷ seems to be justified by the fear that the threat that would be responded to in self-defence would be too vague, opening a ‘Pandora's box’ for any preventive use of force.Footnote ¹⁵⁸ It is therefore possible to argue that no opposition actually exists against – or that there is at least an implied general consensus in favour of – a right of self-defence responding to an imminent threat provided that such a threat is manifestly imminent, that is, has already materialized. This brings us to the second problem raised by Corten's view, which is a problem of legal qualification. Corten univocally qualifies the right to respond in self-defence to a use of force that ‘has materially begun’ as a form of traditional self-defence against an armed attack. This qualification is at least debatable. Nothing indeed prevents qualifying such a right as a particular form of preventive (anticipatory) self-defence, exceptionally permitted. One could object that this last solution is dangerous, since it favours a qualification that would be more easily subject to abuses. Yet, extending the traditional definition of an armed attack without giving any firm legal bases for such extension does entail the same risks. The best approach is perhaps to follow the doctrinal trend and to give to this particular self-defence a sui generis qualification, as ‘interceptive self-defence’.Footnote ¹⁵⁹

A similar problem of qualification arises concerning the right to act in self-defence in order both to respond to past attacks and to prevent the repetition of future similar attacks. If such a right is not contested, provided that the response does not come too late after the attacks have occurred, it seems debatable whether it must be qualified as a form of traditional or preventive self-defence. Corten argues that the particularity of the self-defence action in that case must be analysed through the classical condition of necessity:Footnote ¹⁶⁰ the action, although being undertaken after the attacks have been committed, is still necessary, as it is designed to repel similar attacks that are likely to happen in the near future. As a result, in Corten's view, the right to react to such attacks must be seen as a form of traditional self-defence. Yet, it is also possible to argue for an alternative qualification, preventive self-defence, since states acting in self-defence in this case usually put the accent on the future-oriented aspect of their actions. Moreover, those actions are justified only because the threat of future similar attacks still exists. In any event, the qualification as preventive self-defence has never been expressly excluded in state practice or international case law.Footnote ¹⁶¹ Again, the best solution seems to envisage a sui generis qualification.

The other main issue addressed by Corten with respect to the notion of armed attack is the right to respond in self-defence to an ‘indirect aggression’. After an in-depth scrutiny of state practice and international case law, the author concludes that such a right has not been recognized yet and that self-defence can therefore be exercised only in reaction to a direct armed attack by a state.Footnote ¹⁶² Such conclusion can only be understood in the light of both Corten's own definition of the notion of ‘indirect aggression’ and his own interpretation of state practice. Indeed, the notion of indirect aggression is traditionally conceived as meaning an armed attack committed by a state through its involvement in attacks launched abroad by non-state actors. It is normally used as opposed to the notion of direct aggression, which means an aggression committed by the regular forces of a state. Such interpretation is adopted by many legal scholarsFootnote ¹⁶³ and supported by state practice,Footnote ¹⁶⁴ especially in the debates preceding the adoption of the UNGA Resolution 3314 (XXIX).Footnote ¹⁶⁵ Corten's definition is more restrictive. In his view, indirect aggression refers to the hypothesis in which the involvement of a state in attacks committed by armed groups abroad would be lower than a substantial involvement and would merely consist of, for instance, harbouring those groups on its territory or being unwilling or unable to put an end to their activities.Footnote ¹⁶⁶ Arguing that such minor state involvements cannot amount to an armed attack under the current state of international law, Corten logically concludes that there is no right to respond in self-defence to indirect aggression. One may wonder why the author did not follow another approach that would have consisted of adopting the traditional definition of indirect aggression and analysing whether the scope of this notion had evolved in the light of recent state practice and international case law.

That having been said, it is uncontested that minor involvements of a state in attacks committed by non-state actors, such as the above-mentioned ones, cannot be considered as amounting to an armed attack by this state. This is clearly supported by ICJ case law.Footnote ¹⁶⁷ As far as state practice is concerned, one has already emphasized the questionable interpretation made by the author, according to which several self-defence responses to attacks committed by non-state actors, such as the US Operation Enduring Freedom against Afghanistan in reaction to the 9/11 attacks and the 2006 Israeli intervention in Lebanon in reaction to the rocket attacks by Hezbollah, have to be viewed as responses to an armed attack committed by a state and not as responses to attacks committed by the non-state actors only (cf. subsection 4.1, supra). As a result of such particular interpretation, Corten discusses those precedents at this stage.Footnote ¹⁶⁸ Having analysed in detail the many reactions to those precedents, he concludes that those reactions do not evidence any change in the traditional position according to which only substantial state involvements in attacks committed by non-state actors amount to an armed attack under Article 51 of the UN Charter. Corten also supports this position by analysing the issue through the lens of the articles on state responsibility, particularly the attribution rules.Footnote ¹⁶⁹ He concludes in this regard that minor state involvements in attacks committed by armed groups do not authorize attributing these attacks to the state involved therein and, therefore, considering this state as having committed an armed attack. Although the application of the attribution rules does not seem to contradict the conclusion reached by Corten, especially in light of ICJ case law and state practice, it is not sure, as argued elsewhere,Footnote ¹⁷⁰ that it is relevant to refer to the law on state responsibility in defining a situation whose occurrence is not primarily related to a question of responsibility, but to the right of a state to defend itself against an armed attack.

Having analysed the notion of armed attack, Corten addresses the conditions under which the right of self-defence must be exercised. The first one is provided by Article 51 of the UN Charter, which states that the right of self-defence cannot be impaired ‘until the Security Council has taken measures necessary to maintain international peace and security’. Relying on state practice, Corten argues that the UNSC decisions are ‘superior’ to the exercise of the natural right of self-defenceFootnote ¹⁷¹ but must only be conceived as impairing such exercise and not as putting an end to the right of self-defence itself.Footnote ¹⁷² He also emphasizes three problems in that respect – problems that are well illustrated by references to state practice. The first is determining when the Security Council has taken ‘the measures necessary to maintain international peace and security’, which will impair the exercise of the right of self-defence; this problem, for example, arose with regard to the measures adopted by the Security Council in Resolution 1373 (2001) with respect to the right of self-defence exercised by the United States in Afghanistan in response to the 9/11 attacks.Footnote ¹⁷³ The second problem is determining the extent to which this exercise has been impeded by the UNSC measures; one may mention in this respect the UNSC Resolution 678 (1990) in which the Security Council authorized the use of force ‘unless Iraq on or before 15 January 1991 fully implemente[d]’ the previous SC resolutions. In Corten's view, this prevented the exercise of (collective) self-defence.Footnote ¹⁷⁴ The third problem is determining whether the aggressed state may not respect the UNSC resolution imposing a ceasefire when the aggressor state refuses to abide by the resolution. Corten clearly admits such a hypothesis.Footnote ¹⁷⁵

This analysis of the relationships between the UNSC measures and the right of self-defence is certainly arguable, even if the conceptual distinction between impairing the exercise of the right of self-defence and putting an end to this right would perhaps have needed some more clarification. Such relationships can, however, be analysed from a different perspective. State practice, indeed, seems to reveal two distinct kinds of UNSC measure that may affect the right of self-defence. The first ones are what may be called ‘negative measures’, as they ‘negatively’ affect the exercise of the right of self-defence, such as the decision to impose a ceasefire on states claiming to act in self-defence.Footnote ¹⁷⁶ Such a decision is often imposed by the Security Council in situations in which it is not able to identify the aggressor state or does not want to proceed to such identification in order not to impede the chances of resolving the conflict peacefully. These kinds of UNSC decision are obligatory and states must cease to resort to force even if Article 51 of the UN Charter starts with the words ‘nothing in the present Charter [arguably including the resolutions adopted by the UN organs] shall impair the right of self-defence’. The legal basis for such absolute obligation is actually provided by the final wording of Article 51, according to which the right of self-defence ‘shall not in any way affect the authority . . . of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security’. Since the Security Council has the authority to adopt binding decisions under Article 25 of the UN Charter, this authority cannot be impaired by the right of self-defence. Such reasoning explains the absolute and clear precedence of the UNSC decisions over the right of self-defence. In that respect, it seems logical that, if a state infringes the UNSC decision imposing a ceasefire, the other state will be allowed to respond to it in self-defence.Footnote ¹⁷⁷

The expression used in the first sentence of Article 51 of the UN Charter – that is, ‘until the Security Council has taken measures necessary to maintain international peace and security’ – seems to refer to an entirely different kind of situation: the Security Council has identified the aggressor state and adopts measures in order to substitute its action for the one taken in self-defence.Footnote ¹⁷⁸ The purpose of those ‘positive measures’ is no longer to oppose the claimed right of self-defence, but to take over the exercise of this right, which has not been contested. The question of whether and when those measures imply the end of the exercise of the right of self-defence, as generally raised in legal literature,Footnote ¹⁷⁹ must be analysed in light of state practice as well as the preparatory works of the UN Charter. The former,Footnote ¹⁸⁰ like the latter,Footnote ¹⁸¹ reveals that only the measures that may effectively lead to the maintenance of the peace already restored can possibly have such effect. Yet, in that case, it is highly plausible that the action will no longer be necessary and its prolongation will be unlawful according to the condition of necessity of the law of self-defence. In other words, the expression ‘until the Security Council has taken measures necessary to maintain international peace and security’ does not seem to have any specific effect, distinct from the one resulting from the application of the condition of necessity under the law of self-defence.

The exercise of the right of self-defence must also meet other well-established customary conditions: necessity and proportionality. Corten starts by rejecting some approaches of the notion of necessity as too broad or too narrow before defining this notion and making some links between it and the condition of proportionality. In his view, necessity means neither that the assessment thereof must be left at the entire discretion of the state acting in self-defence nor that all peaceful means must have been exhausted before the action.Footnote ¹⁸² He argues that necessity fundamentally requires an objective assessment of the action in self-defence in relation to the exclusive purpose that such action is authorized to follow, to defend oneself against an armed attack.Footnote ¹⁸³ He then explains the other requirements of the condition of necessity, which are the following: first, immediacy – not too narrowly construed – of the action in self-defence after the past attacks when the purpose of the action is both to respond to such attacks and to prevent the repetition of similar attacks;Footnote ¹⁸⁴ second, effectiveness of the measures adopted in self-defence, this effectiveness criterion being indeed expressly acknowledged in legal literature and state practice while often designated by another expression, adequacy Footnote ¹⁸⁵ or appropriateness Footnote ¹⁸⁶ of the self-defence measures; Footnote ¹⁸⁷ third, the proportionate nature of the action in self-defence.

This last aspect of the condition of necessity is detailed by the author. The main part of the discussion concerns the link between necessity and proportionality.Footnote ¹⁸⁸ Corten asserts in this respect that ‘[p]roportionality . . . seems to be one way to evaluate whether a measure is necessary . . ., [t]he disproportionality . . . be[ing] the mark that the State supposedly acting in self-defence was pursuing some other end than merely riposting to an attack’.Footnote ¹⁸⁹ Such a view may a priori be interpreted as endorsing the solution, which appears to be the most persuasive in relation to the well-known debates on the definition of proportionality. The latter can, indeed, be defined either as requiring a quantitative equilibrium between the action in self-defence and the armed attack (‘quantitative proportionality’), as it is evidenced in state practiceFootnote ¹⁹⁰ and ICJ case law,Footnote ¹⁹¹ or as requiring that the action in self-defence does not exceed what is not necessary to repel the armed attack (‘teleological proportionality’),Footnote ¹⁹² this last approach entirely conflating proportionality with necessity. A convincing answer to this debate seems to consider quantitative proportionality as an easy means for assessing prima facie the necessity of the action in self-defence. This implies that, although it is intrinsically linked to the condition of necessity, proportionality remains distinct from it. Yet, having seemingly argued for such a position, by claiming that proportionality was one way to evaluate whether an action in self-defence was necessary, Corten curiously argues against the independent status of proportionality and seems to conflate it with necessity by stating that ‘proportionality invariably implies comparing the military action justified by self-defence with its essential objectives . . . [and that it] must not . . . be understood as a simple comparison of the material damage caused’.Footnote ¹⁹³ This statement makes Corten's position on the relationships between necessity and proportionality difficult to grasp.

The author identifies another link between necessity and proportionality. Having rejected a too narrow approach of necessity, requiring all peaceful means to be exhausted before acting in self-defence, he nonetheless acknowledges that:

This condition is actually fundamental. It is clearly in line with state practiceFootnote ¹⁹⁵ and legal literature, including the resolution of the International Law Institute on self-defence.Footnote ¹⁹⁶ It entails the exhaustion not of all the peaceful means, but only of all the available practical alternatives to the action in self-defence, including the seizure of the UN Security Council. Although crucial, this condition does not seem to be linked to the condition of proportionality. It is not concerned with the exercise of the right of self-defence, but only with the triggering of such exercise. It is actually one particular aspect of the condition of necessity that should be distinguished from the other above-mentioned aspects, which indeed relate to the exercise of the action in self-defence once this action has been undertaken.

4.6. A right of humanitarian intervention?

The contemporary assertion of a right of humanitarian intervention has been mostly debated in the aftermath of the NATO intervention in Kosovo in 1999. This precedent has given rise to abundant legal literature on the subject, most of the scholars agreeing that, even after Kosovo, no such right had emerged in positive international law.Footnote ¹⁹⁷ This view is completely shared by Corten. The latter persuasively counters all the arguments that have been evoked by statesFootnote ¹⁹⁸ or developed by the minority of scholarsFootnote ¹⁹⁹ in favour of the lawfulness of a right of humanitarian intervention.

The first argument that is refuted is the a contrario interpretation of Article 2(4) of the UN Charter. According to this argument, humanitarian intervention does not fall within the scope of the prohibition on use of force because the latter is considered as prohibiting only uses of force directed against the territorial integrity or political independence of a state or exercised in a way that is incompatible with UN purposes, and that humanitarian intervention is not per definitionem either directed against the territorial integrity or political independence of any state or incompatible with UN purposes, such purposes including the protection of human rights. This argument is comprehensively refuted by Corten.Footnote ²⁰⁰ He could have limited himself to referring in that regard to the preparatory works of Article 2(4) of the UN Charter and the major UNGA resolutions on the use of force. Indeed, it is well known that the final wording of Article 2(4) was included at the request of ‘small’ states in order not to reduce the scope of the prohibition on the use of force, but rather to make it as general as possible.Footnote ²⁰¹ Similarly, the UNGA Resolutions 2625 (XXV),Footnote ²⁰² 3314 (XXIX),Footnote ²⁰³ and 42/22Footnote ²⁰⁴ clearly evidence that the prohibition on the use of force is still conceived as a general prohibition. Corten's response to the a contrario interpretation of Article 2(4) of the UN Charter goes further. In accordance with the Vienna Convention on the Law of Treaties, Corten also rejects this argument by interpreting the contentious words on the basis of their ordinary meaningFootnote ²⁰⁵ as well as the context in which they are provided.Footnote ²⁰⁶ He also refers to a series of declarations in which states expressly reject the a contrario argument.Footnote ²⁰⁷ Finally, he convincingly demonstrates that, although some states proposed to consider the objectives of a military operation as a relevant criterion for qualifying the operation as an aggression, during the debates preceding the adoption of UNGA Resolution 3315 (XXI), this criterion was not accepted by the other states. It is indicative in this respect that the resolution expressly provides that ‘[n]o consideration of whatever nature, whether political, economic, military or otherwise, may serve as a justification for aggression’.Footnote ²⁰⁸

Another argument supporting the lawfulness of the right of humanitarian intervention could be based on the notion of responsibility to protect (R2P), which has recently been forged by the Commission on Intervention and Sovereignty of States. This notion basically means that any state has the primary responsibility to protect its population from genocide, war crimes, ethnic cleansing, and crimes against humanity, and that, in case such responsibility is not (adequately) exercised, the international community is entitled to intervene to protect the threatened populations.Footnote ²⁰⁹ Yet, as compellingly shown by Corten, neither the declarations made by the states in relation to that notion before and after the report of the Commission nor this report and the 2005 World Summit Outcome, which briefly refers to the R2P,Footnote ²¹⁰ involves any recognition of a right of humanitarian intervention.Footnote ²¹¹ Only very few states declared themselves in favour of such a right while the Commission was working on the R2P.Footnote ²¹² Besides, those declarations were apparently made de lege ferenda and even fewer states maintained such a position in their declarations expressed in the context of the World Summit.Footnote ²¹³ More fundamentally, the report of the Commission itself and the World Summit Outcome do not in any way entail the recognition of a right to use force beyond the traditional conception of the matter – that is, a general prohibition on the use of force with only two exceptions: self-defence and force authorized by the UN Security Council. This new concept and the debates surrounding it even seem to have reinforced such traditional conception.Footnote ²¹⁴ The recent military operation in Libya (2011), whose official objective is to protect civilians against attacks, is in line with Corten's conclusion. Although the notion of R2P, as endorsed in the World Summit Outcome, was part of the arguments put forward by states to support the launching of the operation, those states sought an authorization to use force by the UN Security Council.Footnote ²¹⁵ This authorization came with UNSC Resolution 1973 (2011). The same thing may be said with respect to the recent French and UN military operations in Ivory Coast. Those operations, although referred to by some states as undertaken in the context of R2P,Footnote ²¹⁶ were authorized by UNSC resolutions.Footnote ²¹⁷

The final argument that had to be countered was the argument based on state practice. It was not difficult for Corten to object to such argument in light of his positivist methodological approach regarding state practice. Indeed, most of the precedents that are invoked in legal literature are not relevant, since the state's having used force did not justify it by relying on a right of humanitarian intervention, but invoked another, more classical, legal basis.Footnote ²¹⁸ It is not helpful either to refer to precedents in which such a right seems to have been argued, since the declarations of the intervening state are often unclearFootnote ²¹⁹ or explicitly affirm that the intervention must not be counted as a precedent contributing to the emergence of a general right of humanitarian intervention.Footnote ²²⁰

Corten's demonstration with regard to the right of humanitarian intervention is particularly convincing. It seems incontestable, at least from a positivist perspective,Footnote ²²¹ that such a right does not exist in the current state of international law. As demonstrated in this last chapter, such a conclusion also applies to the claimed right to use force in order to rescue nationals in the territory of another state other than in self-defence.Footnote ²²² None of the above-mentioned arguments, which could be invoked in support of such a right, is founded. Particular attention is paid to the debates in the context of diplomatic protection. Corten clearly demonstrates in this regard that no right to use force for rescuing nationals abroad on another basis than in self-defence was finally recognized during these debates, the final outcome of which even seems to reject such a right.Footnote ²²³