It would not be an overstatement to assert that the publication of the first scholarly commentary on the two Vienna Conventions on the law of treaties is a significant milestone. Aside from the commentary of the International Law Commission adopted on the completion of draft articles of the two conventions,Footnote ¹ there was no comprehensive scholarly commentary on these two foundational texts of the international legal system, something long lamented by both scholars and practitioners. Such a lack was particularly bewildering, as almost all other significant contemporary conventional instruments had already been subject to a collective commentary by scholars.Footnote ²

Convinced that 37 years after the signature of the Vienna Convention on the law of treaties between states (and 26 years after its entry into force)Footnote ³ as well as 20 years after the signature of the Vienna Convention on the law of treaties between international organizations and between states and international organizations,Footnote ⁴ such a lack had to be addressed, Professor Olivier Corten and Professor Pierre Klein, together with the International Law Centre of the Faculty of Law of the Free University of Brussels (ULB), ignited the bold enterprise of collecting the contributions of as many as 80 authors to carve out a joint commentary on these two fundamental conventional instruments. The realization of this wide-ranging project stretched over more than five years and was concluded by a conference held in Brussels in October 2006 on the occasion of the publication of the three-volume commentary by Bruylant publishers.

Since the work is edited by two leading French-speaking international legal scholars, it is not astounding that the commentaries on each of the articles of these two conventions have been mostly written by eminent French, Belgian, and Swiss authors (see in particular A. Pellet, J. Verhoeven, H. Ruiz-Fabri, J.-M. Sorel, J.-P. Cot, J.-M. Thouvenin, J. Salmon, E. Suy, E. David, P. Daillier, M. Cosnard, H. Ascensio, P. Couvreur, N. Levrat, L. Boisson de Chazournes, R. Kolb, C. Dominicé, E. Wyler, and L. Caflisch to name only a few). It must however be stressed that many other celebrated international scholars have contributed to this enterprise (see, among others, B. Simma, P. Sands, M. Shaw, C. Tomuschat, G. Gaya, M. Kohen, P. Kovacs, W. Schabas, M. Bedjaoui, D. Turp, etc.), thereby shrewdly playing down the French–Belgian–Swiss stamp on this work. Klein and Corten deserve some additional praise for venturing to entrust young promising authors (such as F. Dopagne, C. van Assche, A, Langerwall, C. Denis, F. Roch, etc.) with the writing of the commentary on some important provisions. Other contributions include those of confirmed young academics or practitioners, such as P. d'Argent, C. Tams, C. Laly-Chevalier P. Gautier, and N. Angelet, who have long won their spurs in international legal scholarship.

As far as the structure of these volumes is concerned, it is appropriate that the 1969 Convention on the law of treaties between states and the 1986 Convention on the law of treaties between states and international organizations and between international organizations be commented on together and side by side. Thanks to the modelling of the 1986 Convention on the blueprint of the 1969 Convention, corresponding provisions of the two texts are examined alongside each other. The reader is thus able to appraise the extent to which the provision of one departs from the corresponding provision of the other and can, accordingly, single out all the peculiarities of each of these two treaty regimes. The commentary of each provision also rests on an astute common three-tiered structure. First, an account of the general features of the article concerned is provided (including a study of the object and purpose as well as an analysis of the normative status of the provision concerned). A second part is devoted to the interpretation of the provision. It is followed by a third part grappling with the effects of the provision. The analysis carried out according to this three-tiered structure borrows substantially from the work of the International Law Commission and the travaux préparatoires of the Vienna Conventions. But it has also extensively resorted to the subsequent practice of states and international organizations and the relevant case law – what is naturally of paramount importance for appraising the customary status of each provision.Footnote ⁵

It is not contested that a very large number of provisions of the two Vienna conventions on the law of treaties have turned out to be customary international law. Differences between the two conventions may, however, arise in this regard, as one can probably argue that more rules of the 1969 convention have achieved customary status than those of its sister 1986 convention. Quite apart from the fact that the former preceded the latter by almost twenty years, it can be posited that the slightly different customary status between the rules of each of these two conventions is not utterly alien to the non-justiciability before the International Court of JusticeFootnote ⁶ and other international tribunalsFootnote ⁷ of cases whose ‘very subject matter’Footnote ⁸ pertains to the responsibility of international organizations, as this lessens the extent to which the law of treaties between international organizations or between states and international organizations is applied by international judges. This is not to say that customary international law boils down to the arbitrary creation of international judges.Footnote ⁹ However, the practice has demonstrated the sweeping contribution of international courts in the identification, if not the emergence itself, of customary international law which usually remains shrouded in some sort of uncertainty until it is applied by an international judge. Be that as it may, even if these two conventions in their entirety one day reflect customary international law, Corten and Klein's commentary would remain an enlightening and useful guide through the interpretation and the understanding of the rules governing the law of treaties as treaty law remains an inescapable tool for interpreting corresponding customary international law.Footnote ¹⁰

The customary status of a treaty provision is intrinsically – although not exclusively – linked to the extent to which the corresponding rule is applied in practice. In that respect, one cannot help emphasizing that some provisions are being more applied than others. One may refer, for instance, to those provisions related to the interpretation of treaties (Arts. 31 ff.).Footnote ¹¹ That does not necessarily mean that the most applied provisions are those which, once adopted,Footnote ¹² have attracted most attention in the literature. Paradoxically, the fiercest doctrinal controversies pertaining to the law of treaties have mostly revolved around those least applied provisions, as illustrated by Article 53 or 64 of the Conventions relating to invalidity and termination in case of conflict with jus cogens.Footnote ¹³

The foregoing may not be true as regards the rules pertaining to the reservations to treaties (Arts. 19 ff.), which deserve a few specific remarks here. These rules, and the corresponding provisions of the Vienna conventions, have triggered a dramatic scholarly debate since their inception, while the uncertainty besetting them has not prevented their wide application in practice. Their incompleteness, if not their internal incoherence,Footnote ¹⁴ has prodded the International Law Commission to return to the topic.Footnote ¹⁵ Although the 1995 preliminary conclusions on reservations to normative multilateral treaties adopted by the Commission have proved valuable,Footnote ¹⁶ the final outcome of the work of the Commission on the question of reservations to treaties – in the form of a ‘guide to practice’Footnote ¹⁷ – will fall short of designing a watertight set of provisions addressing the lingering inconsistencies of the provisions of the Vienna conventions on this point. In the absence of any clear rules on this matter, there is a high probability that the problem caused by the reservations to treaties will not abate in the future. The persisting uncertainties of the legal regime of reservations to treaties will undoubtedly inflate the value of the commentary on the two articles 19 which can be found in this volume, thereby underpinning the idea that the commentary on articles 19 constitutes one of the most important contributions to this collective work. There is no surprise that such a difficult and important task had to be bestowed on a scholar whose mastery of the question of reservations was unchallenged. Thanks to his knowledge, expertise and acumen, the special rapporteur of the International Law Commission was somewhat inevitably scheduled for that job. However, some readers may wonder whether an author endowed with more hindsight would have been preferable as the views of the special rapporteur are already well known due to his 13 reports on the topic and his grip on the debate well embedded. Moreover, the reader will not fail to notice that the contribution of Alain Pellet on this point is, by far, the lengthiest and bulkiest of all the commentaries contained in these three volumes. Despite his extensive writing on the topic, the special rapporteur of the International Law Commission has not been deterred from writing 148 pages just for Article 19 of the 1969 Convention.Footnote ¹⁸ Such lengthiness may be perceived as an indication of the importance, in the eyes of the editors of these volumes, of Article 19 of the 1969 Convention. This surely is not far-fetched against the backdrop of the aforementioned lingering complexity of this question and the ongoing work of the International Law Commission. It may well be that the latitude and room granted to such a scholar who had already had sweeping opportunities to express his views on the topic also betray the difficulty Corten and Klein had in reining in such a prolix and influential scholar . . .

The significance of Corten and Klein's volumes eventually stems from the types of rule that are the object of such a commentary. The rules governing international conventions, like the principles governing the international responsibility of statesFootnote ¹⁹ and international organizations,Footnote ²⁰ constitute the bulk of what scholars commonly dub the ‘secondary rules of international law’.Footnote ²¹ These are those rules that make international law a ‘system’. Given the key role played by the secondary rules in the international legal order, and especially the law of treaties, there is little doubt that a commentary on the law of treaties will prove to be an influential tool for scholars, practitioners, and students. This will only be the case, however, as long as the user of this commentary is endowed with a fair command of French. It is worthy of mention that the first commentary of these two essentials texts of international law has been written in French. Indeed, at a time when international legal scholarship is overwhelmingly English-speaking, it is remarkable that, on this very essential topic of international law, the lead has been taken by the French-speaking authors, which reminds us of an epoch when French was the single language of international law and of its doctrine. The French-speaking authors demonstrate here that, even if their language has been dwarfed by English in contemporary legal scholarship, they can still shed light on international legal thought. This being said, the influence of the French literature should not be overestimated. For many scholars around the world, the French scholarly writing remains very arcane, if not inaccessible. Although mistakenly, many scholars understand it to be the embodiment of a fusty positivist doctrine that held sway until the middle of the last century.Footnote ²² These scholars and practitioners will continue to call for an English-language commentary on the Vienna Conventions. This is one of the reasons why an English-language commentary will no doubt be achieved some day. In the meantime, this French commentary on the Vienna Conventions on the law of treaties will constitute the work of reference on this subject matter.

That the commentary in French on a major international treaty precedes the English one is not unprecedented. Indeed, the first commentary on the UN Charter was written in French before being capped by a commentary in English almost a decade later.Footnote ²³ Although the contingency could explain this renewed advance of the French-speaking doctrine, the idea that systematic commentaries on legislation are more in line with continental and civilist traditions is not so far-fetched. The adoption of a rigorous systematic and legalistic approach remains one of the hallmarks of the French-speaking doctrine. This is not to say that French-speaking doctrine is entirely uniform. It is, however, less fragmented than the dominant English-speaking scholarship, probably for reasons both directly and indirectly pertaining to its size. However diverse French-speaking scholarship may ultimately be, there is no doubt that having a French commentary on the text of the Vienna conventions on the law of treaties contributes to the diversity and plurality of international law as a whole, thereby playing down its portrayal as an hegemonic project. For that reason too, this collective work must be warmly welcomed.