Just War in the Middle Ages

The origins of what became the tradition of just war set a framework for later developments in morality, law, and political order. This was not an abstract body of moral theory; nor was it a moral frame superimposed onto reality. Rather, just war thinking took shape as a practically informed morality of war, drawing on church law, theology, Roman law, and the practice of government and war.

Though the just war idea has roots extending back into antiquity in the West, the first comprehensive, systematic conception of just war was defined over a century and a quarter during the Middle Ages in a context characterized by widespread disorder and armed conflict. The most substantive work was done by a succession of canonists beginning in the mid-twelfth century with Gratian's Decretum, whose original title, Concordance of Discordant Canons, well reflects the state of church moral teaching in the West at the time. Gratian changed this, drawing together related wisdom out of a canonical tradition reaching back to the era of the fall of the Roman Empire and applying it to specific moral subjects, among them war. Two generations of successors refined Gratian's effort and added to it, producing a result that Thomas Aquinas succinctly summarized in the Question “On War” in his Summa Theologiae, completed in 1274. Though Aquinas's work is much better known today, that of the canonists was both more basic and more comprehensive. While Aquinas focused on what we today would call the jus ad bellum (a modern legal term)—the conditions for justified resort to war—the canon law also included rules relating to the conduct of war, specifically noncombatant immunity and limits on certain kinds of weapons. The canonists' work on just war went on simultaneously with the recovery and interpretation of Roman law (also involving canonists), including the ideas of the law of nations (jus gentium) and natural law (jus naturale), both of which were important in the construction of the just war idea.

A prominent theme in both just war thinking and the recovery of Roman law was the establishment of order. The collapse of the Western Roman Empire several centuries earlier had left a pattern of regional hegemonies different in size, cohesion, strength, and legitimacy. These regional political communities were organized on the lines of older relationships that were in the process of coalescing into the feudal system of interlocking dependencies and responsibilities. The nominal ruler of each such community could claim the title of princeps (prince), denoting the principal governing authority, but powerful individuals who bore other titles—duke, count, earl, baron, and on down to knight, each with its own history and basis for a claim to ruling authority—might not support the prince but might fight among themselves and with the prince to serve their own authority and power. Princes thus had to deal with not only rivalries with other princes but also substantial internal conflicts, with every regional authority claiming the right to use armed force (that is, the right to bellum, war) to enforce his own claims. The church, though nominally universal (the meaning of the term “catholic”) and unified under a single head (the Pope), was in reality similarly unsettled, reflecting the influence of the various subcommunities within the organization and the power of their clergy. Likewise, the relationship of the church's authority to that of the temporal sphere was similarly unclear and marked by tension.

The canonical definition of bellum justum (just war) responded to all these elements of uncertainty and conflict in the historical context, including both the relation between churchly and temporal authority and the disputes among temporal authorities that plagued individual political communities. To address the former, the canonists cited a fifth-century papal letter to the Roman Emperor of the time in order to draw a distinction between the authority of temporal rulers and the spiritual authority of the church, giving autonomy and differentiated responsibility to each in its own sphere. From this basis they restricted the authority to use armed force to the sphere of temporal government, denying the right of the spiritual authorities to do so. Though they acknowledged a universal right to use force in self-defense against an attack while the attack is in progress, use of force after an attack to correct a wrong done was another matter. The canonical conception of just war restricted the right to determine when justice has been violated and to employ armed force to correct this to each temporal ruler with no temporal superior (the princeps or, in a term just coming into use in the French vernacular of the period, the souverain, sovereign) in each political community, thus delegitimizing the claim of other authorities to be able to resort to arms to settle disputes or further their own purposes. In cases of disputes of justice the prince was charged to act as judge of last resort, determining what natural law required in the case, and then to correct any violation of justice, including punishing the wrongdoers, using armed force if necessary. Finally, in using armed force the prince was to act with right intention, which the canonists defined negatively by reference to a list of wrong intentions (drawn from Augustine), and positively (again by reference to Augustine) as the intention to restore peace within the political community. In practice this meant the prince had the right to use armed force only to correct a violation of justice, so that the prince's authority to use armed force depended on a just cause and had to be undertaken with right intention.

These three criteria for a just war—sovereign authority, just cause, and right intention—fitted tightly within a larger conception of politics, as they corresponded directly to the three classical goods or ends of politics drawn from Roman law: order, justice, and peace. The understanding of just war thus defined directly addressed the widespread problem of domestic lawlessness and disorder, bringing to bear both church tradition and Roman law so as to strengthen the role of the supreme temporal authority in each political community and undergird the ruler's responsibility to establish these ends in discharging the responsibility of governing. The assumption was that if each sovereign diligently fulfilled the responsibilities of rule, different political communities would not come into conflict with one another. But where the injustice providing just cause for resort to war came from the domains of another prince, war between princes became a possibility, with the rectification of injustice understood to be a matter of the outcome. This idea, though, was drawn from another source of norms in medieval European culture—the Germanic warrior traditions—that also led to the chivalric code and became incorporated into the jus in bello of the developing tradition on just war.

The original systematic conception of just war thus had to do not only narrowly with the use of armed force but also with government, the responsibilities of rulers, the purposes of political life, and the relationships between and among political communities. It was, as already noted, a practically informed morality of war. Though the authors of this conception were men of the church, they framed just war around the responsibilities of temporal rule as defined by the natural law and as expressed in the customs and practices (the jus gentium) of each political community and of the larger community formed by the interrelationships among particular political communities. The result included elements drawn from church tradition, Roman law and custom, Germanic traditions relating to rule and to war, and recognition of customary practices and responsibilities of members of all ranks of society. Taken as a whole, this conception of just war expressed a cultural consensus on world order that held until well into the modern period, when changed circumstances led to modifications in both.

Changing Just War: The Fifteenth to Seventeenth Centuries

The full story of these changed circumstances, the reactions to them, and the resultant modifications includes the consolidation of European monarchical states, the encounter of Europeans with the New World, the fracturing of European cultural unity by the division of the Western church, and the century of religiously linked warfare that ended only with the conclusion of the Thirty Years’ War in 1648. Major theorists who contributed to rethinking the conception of just war and its implications include the theologian and political adviser Vitoria in the fifteenth century, the legal theorist Gentili in the late sixteenth century, and the early seventeenth-century thinker Grotius, who, though he made his mark mainly in law, was also trained theologically. For the present context, a focus on the contributions of Grotius serves both to mark the changes in the just war idea during this period and to define the lines of development in warfare and political order going forward in time.

Grotius, like Vitoria and more than Gentili, assumed the tradition of just war and worked from it; yet in response to the challenges of his time he recast the just war idea he inherited in fundamental ways, importantly influencing subsequent thinking about war and the shape of international order.

First, in the context of the Dutch War for Independence from Spanish rule, Grotius redefined the sources, nature, and responsibilities of sovereignty away from those embodied in the medieval canonists' conception of princely rule. The medieval thinkers had assumed a prince's rule to be legitimate except in cases of behavior that turned him into a tyrant; even then, as Aquinas cautioned, care should be observed in removing a tyrant, because this could make matters worse. Grotius's thought, however, moved sovereignty away from the prince, locating it instead in the people of the political community, the laws and customs passed down in their traditions, and the territory they inhabited. The governing authority of the ruler, on this conception, depended on the community's sovereignty thus defined. Any effort to exercise rule on another basis was inherently unjust, and rebellion therefore was justified. For the specific context of war, any use of armed force or political pressure that violated the borders of the political community inhabited by a discrete people also violated their fundamental rights and the ancient laws and customs of that community. Aggression in the form of violation of a political community's borders thus became the central feature of the idea of just cause as conceived by Grotius. This was to have a major impact on international law, extending ultimately into the United Nations Charter and contemporary world order.

Second, Grotius, for his own reasons, took seriously the possibility that in war both belligerents might have just cause, so far as they or any neutral observer could tell. I have called this the possibility of “simultaneous ostensible justice.” For Vitoria, who had also noted this possibility, this implied that belligerents should prosecute the war scrupulously according to the restraints of jus in bello. While Grotius argued this way as well, he took the matter somewhat further, refocusing the inherited tradition on just war to emphasize the standards to be followed in fighting—standards he regarded as established by long-standing custom, but also recognized in the common law among European societies, which he understood ultimately to reflect the Christian ideal of charity. Grotius's thinking on the consensual limits to conduct in war was, for practical purposes, the beginning of the idea of a law of armed conflicts as rooted in European cultural standards. It was in this way—in his understanding of the term jus gentium and that of thinkers who built on his thought—that the “law of nations” was arrived at by common consent.

On the matter of resort to war, the wars of religion—including the Thirty Years' War, which was ongoing as he was writing his major work, De Jure Belli ac Pacis (On the Law of War and Peace)—demonstrated all too clearly the problem of a conception of the right to war based on a sovereign ruler's determination of a violation of justice, for in the wars of religion dissent from the ruler's religion became such a violation. Grotius's reconception of sovereignty refocused just cause to a violation of a political community's territorial border. Such a violation was an objective fact, not a matter of moral judgment by the ruler; and accordingly for Grotius, the ruler's role was not to provide such a judgment but rather to make a formal declaration of the state of war, listing the objective violation or violations that gave rise to it.

Into the Twentieth Century

All these ideas became part of the European world order that coalesced in the wake of the Peace of Westphalia (1648), ending the Thirty Years' War, aided by development of the idea of the law of nations by Grotius's successors, most prominently Pufendorf, Wolff, and Vattel. Monarchical rule remained a feature of this order, though it coexisted with a conception of sovereignty defined by territory and rooted in a nation's people and their particular laws and customs. Other thinkers, including Locke and some of the French philosophes, added an emphasis on human rights as possessed by each individual and expanded the importance of popular assent as essential to government. In turn, the American and French revolutions established institutional embodiments of this conception, affecting international order accordingly.

Though a broadly consensual conception of the law of nations persisted through these changes, the origins of positive international law in the sense understood today lie specifically in the arena of warfare, more specifically in a number of agreements reached during the late nineteenth and early twentieth centuries aimed at controlling the prosecution of war and mitigating its harm. During the American Civil War, General Orders No. 100 (1863) established what would today be called “rules of engagement” for the Union army on the basis of the consensually understood “laws and customs of war.” While this was not an international agreement, its content influenced later international agreements on war. Almost simultaneously in 1864 came the adoption of the first Geneva Convention, an international agreement setting standards for the care of the wounded in armies in the field. A series of international agreements of various sorts followed, including the 1868 St. Petersburg Declaration banning the use in war of explosive projectiles under 400 grams in weight; the 1899 Hague Declarations renouncing the use in war of asphyxiating gases and expanding bullets; and, most prominently, the various 1907 Hague Conventions, including the Annex to Convention IV, “Regulations Respecting the Laws and Customs of War on Land”; as well as several conventions specifically addressing aspects of naval warfare. All these early steps were understood as building on the foundation of the “laws and customs of war,” as the Preamble to 1907 Hague Convention IV makes explicit: the aim was “to revise the general laws and customs of war, either with a view to defining them with greater precision or to confining them within such limits as would mitigate their severity as far as possible.”Footnote ³ Early positive international law on war was thus understood to be built on the basis of consensual “laws and customs,” and these in turn reached back through Grotius and his successors to the jus in bello of just war tradition. Subsequent international law—both that specifically focused on war and that having to do with other issues, including the formal definition of international order in the Covenant of the League of Nations and the Charter of the United Nations—no longer takes note of this historical and thematic connection to the tradition of just war, but nonetheless this connection remains, and the laws of armed conflict as well as the shape of world order carry forward elements from that tradition. Contemporary international law and world order have also been influenced by traditions and values from other cultures, and I return below to what this implies.

Contemporary World Order

Against this background, then, what can be said about changes in world order and the revisions these may imply for international law and thinking about just war?

World order is always changing, and indeed it has evolved so as to accommodate changes. It is a complex concept, with the legal and institutional structures associated with the United Nations defining its core, but other factors also playing important roles in filling out its full shape. Such factors include global and regional economic relationships, interactions over energy and related questions of development and climate change, the capabilities of contemporary transportation, the revolutions in communication and information technology, and the relation between local and regional cultures and a spreading global culture. Far from inconsequential, furthermore, is the role of individual states within world order. All of these relationships, including the United Nations system itself, depend fundamentally on agreements among individual states, and each state brings to every one of these relationships its own interests, its own cultural assumptions, its own aspirations—collectively, the components of its own conception of sovereignty.

Most often when changes occur in one or another of these factors, while the particulars of international order are affected, the overall order itself remains stable. Other kinds of changes, though, are more serious and threaten the very structure of the international order. Large-scale armed conflict is such a change, laying bare existing tensions in the relations among affected states and their populations, creating and aggravating others, and imposing stresses on the institutional and legal structures designed to limit the outbreak of such conflict and restrain it once it has begun. Both of the major efforts to create an international structure of world order, the League of Nations and the United Nations, were crafted in response to major international wars, and both were designed to limit resort to war among their members—the former by emphasizing arbitration to settle disputes, the latter by adding a pledge renouncing first resort to armed force in disputes between or among its members. The approach to avoiding war taken in Article 2 of the UN Charter has held up well, though it has been tested at various times in various contexts. The overall lesson of its history seems to be that violation of the nonaggression rule does not in itself threaten world order; that depends more broadly on how any given violation is treated by other actors, especially in its aftermath. The full dimensions of this lesson continue to unfold, and they may be different in every new case.

The rise of nonstate actors in world affairs creates a significant challenge to world order and to international law. The United Nations system of world order, together with the associated body of positive international law, is built upon the existence of independent states and their mutual consent to being interrelated within that system. In this conceptual, legal, and institutional framework, nonstate actors have no natural fit, and their very existence challenges the order built on states and their interrelationships. Article 2 of the UN Charter attempts to prevent the initiation of war by one state against another, but it does not address civil wars, rebellions, or terrorist activity, and these have comprised the majority of armed conflicts over the last fifty years. Similarly, international law on conduct in war has almost entirely been framed in terms of interstate warfare, and the one exception, 1977 Geneva Protocol II relating to the Protection of Victims of Non-International Armed Conflicts, is limited by its brevity and relative lack of coverage.

The system of world order based in the United Nations has failed to prevent armed conflicts involving nonstate actors; and while such conflicts create local and regional disorder, in general they have not threatened world order. The one important exception is armed conflict rooted in radical Salafist Islamic militancy, whose guiding ideology aims to establish an entirely different form of world order, one based in its understanding of the requirements of faithfully Islamic world governance: a universal caliphate.

Through the law of armed conflicts and by outlawing genocide and crimes against humanity, international law seeks to stabilize world order by defining what is legitimate in the use of armed force. Correspondingly, the threat to both the structure of the law and to world order lies in the breakdown of the restraints thus established. Like other elements of international law, observation of the restraints it imposes on conduct in the use of armed force depends fundamentally on the willingness of states to enforce proper behavior in their militaries. In particular cases, though, states may fail to live up to their obligations in the law. Sometimes states themselves, through government policies and decisions, have been principal offenders, while weak or failed states involved in armed conflicts have lacked the internal resources to ensure compliance with the established restraints on the military forces operating within their borders. Further, even otherwise well-constituted and well-disposed states may fail to establish sufficiently robust systems for command, control, and discipline of their militaries. Stresses to this regime of restraint laid down in international law have in the past arisen for all these reasons.

The above comments apply to conflicts involving states as well as conflicts involving nonstate actors. But a heavier burden of stress imposed on the effort to restrain war by international law comes from the fact that much recent armed conflict has involved nonstate groups on one or both sides. How far the aforementioned restraints apply to such groups remains unsettled, and in practice these groups have fought in ways that have ignored them. Both world order and international law are challenged by these recent and ongoing patterns in warfare, and exactly what this may mean for the future remains unclear.

One recent development designed to deal with conflicts involving both state and nonstate actors was the establishment of the International Criminal Court (ICC). Since its creation the ICC has provided a continuing trans-state mechanism for identifying, prosecuting, and punishing violators when their states have not done so, and the reach of the Court extends to heads of state and other government officials who may be among the violators as well as to nonstate groups and individuals. But the context in which offenses against international law take place inherently makes prosecuting offenders extremely difficult. In some cases it may not be possible even to identify them; in other cases persons identified for prosecution may be protected by friendly states or by the nonstate groups to which they belong; in virtually every case, gathering evidence and securing credible witnesses is difficult and may be impossible; and overarching all this are the limits of time, funding, and other resources needed to complete every prosecution. The ICC constitutes one kind of response to violations of relevant international law and the stresses and challenges to world order imposed by the nature of recent and ongoing armed conflict, but it provides no universal remedy.

Another kind of response directed to warfare by nonstate groups is the various proposals to effectively change the law on what is allowed in armed conflict so as to legitimize, or at least tolerate, practices used by nonstate groups that violate existing international law. I have serious reservations about this approach, as I have argued in a recent review essay in this journal.Footnote ⁴ The case of radical Islamist militancy poses particularly pointed challenges to international legal efforts to restrain the use of armed force. While there is no real prospect for those adhering to this ideology to achieve their ultimate aim of changing the world order, their actions bring very real danger. This includes groups such as the Islamic State in Iraq and Syria (ISIS) and al-Qaeda–related organizations in the Islamic Maghreb, Yemen, and Somalia, as well as individuals who take their lead from these groups and their common ideology. Their actions, which include both acts of terrorism and organized military activity, openly flout the parameters set down in international law regarding genocide, crimes against humanity, and the law of armed conflict. Since it is the law itself that is rejected, the goal of these groups and affiliated individuals is to do away with that law and replace it with their own.

My own approach to these contemporary challenges is to hold fast to the existing restraints and seek ways to strengthen them, simultaneously also seeking to strengthen the world order of which they are a part. To my mind, the most promising path toward doing this, as already suggested, is to take more seriously the culturally based moral underpinnings of both international law and world order as a way to fill out the underpinnings of the international consensus that is necessary for both. As noted earlier, positive international law, including the law of war, historically developed out of the “laws and customs of war” as well as the assumptions and patterns of international behavior in the West. These reached back politically through the Peace of Westphalia and intellectually through the work of Grotius to the tradition of just war. Historically and thematically, then, the systems of international law and world order are connected to the idea of just war as it took its classical form during the medieval period and developed further in the transition to the modern era. Recovering this moral underpinning is an important part of strengthening both international law and world order in the face of present-day challenges and stresses. But to do so would be sufficient only if the contemporary world order and positive international law were still defined in terms of the interactions among Western states and the traditions and values held in common by them.

In the contemporary global context, though, this represents only a part of the picture. Today, both international law and the system of world order are understood as defined by the choice of states to give formal agreement to them. This provides a way to create a functional framework that brackets-out the civilizational and cultural differences among the participating states. Yet the moral perspectives developed in each of these cultures remains; and just as with the West, to recover these moral underpinnings would add a deeper dimension to the mutual participation of states in world order and international law.

Over a half century ago Quincy Wright observed that every major culture has produced its own morally informed conception of good government, right relations among states, and ways to conduct war, noting also that wars across major cultural boundaries tend to be the hardest to restrain precisely because of the deeply rooted differences between them. More recently, Samuel Huntington has offered a similar analysis, calling for better knowledge across civilizational borders as a way to mitigate or prevent clashes rooted in civilizational differences. An attempt to recover the historical moral and thematic links between just war tradition and international law thus should be accompanied by similar work on the moral traditions related to government and war in other cultures and by comparative efforts to find commonalities and agreements across these different traditions. I suggest that in the contemporary world the moral traditions that are particularly important to seek to understand and engage, alongside that of just war, are those of Islamic culture and those of Chinese culture.

Until fairly recently, the kind of cross-cultural dialogue I am advocating would not have been possible. While there has now been a century of scholarship on Western tradition on the ethics of war and its relationship to international law and world order, it is only in response to the challenges posed by the rise of radical Islamist militancy that scholarship has developed that focuses on investigating and understanding Islamic traditions on government and jihad of the sword. Even more recently, the growing prominence of China in global international relations has also fed scholarship on Chinese traditions on war and government. What is now known about these two traditions as well as that of just war provides a substantial base for a comparative effort aimed at uncovering areas of agreement and using them so as to fortify bonds of linkage in world order as well as to strengthen challenged areas in international law, especially pertaining to the conduct of armed conflict. Understanding these cultural traditions and finding commonalities among them provides a potentially helpful path to navigating the changes in world order, international law, and the ideal of just war as history moves on.