Two of the most obvious facts of our political life today are the persistence of warfare and the centrality of the sovereign state. The rise of regulated war from rampant violence and the emergence of the state from a former world of empires seem to have evolved hand in hand. Roman jurists were among the first to have both formulated ideas about the morality of war and attempted to circumscribe the range of justifications for permitting certain kinds of violence. Sovereignty, in the sense of state authority with powers of legitimation over the laws of war, emerged much later and historically arose as coterminous with modern statehood, and it is therefore largely a product of the seventeenth and eighteenth centuries. The political thinkers associated with the early modern period took a view of war between sovereigns as possible—and sometimes necessary—to be waged legitimately, and inserted it into their view of domestic politics. The jurisprudence of war that emerged from their writings continues to shape how we understand the laws of war today, not only as a component of international law but also as a distinctive branch of ethical conduct concerned with the rights and wrongs of war.

To speak of the laws of war, then, is also to confront their Janus-faced nature: legally, they legitimize state force, whereas ethically, they regulate belligerence. Pablo Kalmanovitz’s The Laws of War in International Thought focuses especially on the latter: how to think ethically about war. This question, both as a matter of philosophical inquiry and political expediency, still remains woefully neglected in contemporary scholarship on just war. With rigor and elegance, Kalmanovitz approaches the subject theoretically and (to a lesser degree) historically as he seeks to speak to three audiences simultaneously: intellectual history, international political thought, and critical history of international law. Conceived as a study of the “intellectual history of the laws of war before their codification,” the book does not seek to settle long-standing debates over the ethics of war but to illuminate how present-day arguments concerning the laws of war stem from “particular vocabularies” inherited from past political and legal thinkers (p. 3). Meticulously crafted in its contextualization of ideas and deeply attentive to the dynamic between moral choice and military necessity, Kalmanovitz’s book shows how the theoretical construction of state sovereignty is inextricably linked to a defense of the legalization of the laws of war. As a scholarly intervention in the broader literature on the formative period of modern theories of international law, it reads as a continuation of a debate that could be said to have begun about two decades ago with the publication of Richard Tuck’s The Rights of War and Peace (1999).

Unlike much of previous scholarship, which has traditionally focused on interstate relations as generative of modern liberal political theory, Kalmanovitz’s The Laws of War in International Thought presents arguments over the moral basis for war and international aggression. Central to the book’s intellectual trajectory is an understanding of “regular war,” which can be delineated in the works of humanist writers. Rooted in the ideas of Roman jurists, as they gained widespread respect in fifteenth-century Italy, the jurisprudence of war that emerged in humanist thought was less concerned with the legitimacy of war as such, because few would deny that some violence is not only permitted but even necessary. Instead, the idea of “regular war”—as it would eventually come to be called—centered on the range of its permissibility and the justification of its use morally. The justness of war was subsequently displaced by the legitimacy claim: what makes a war regular is the positioning of the belligerents not as parties committed to the cause of justice but rather as enemies who stand in a relation of legal equality. Because their differences may not be reconciled by any other means, such equal rivals choose to resort to war as a tool to settle their dispute. As a major thread interwoven in Kalmanovitz’s book, regular war is examined in its evolution from early modernity to the present day and treated extensively in its various aspects of ad bellum, in bello, and post bellum.

For a book of relative brevity, The Laws of War in International Thought accomplishes in its five chapters its ambitious goal to show how central the idea of state sovereignty was (and continues to be) in regulating war through law. Chapter 1 introduces the reader to the theological problem of conscience and how the Salamanca school of the Spanish scholastics—including Vittoria, Suarez, Molina, and Vasquez—attempted to address it. Because they did not treat the question of moral conscience separately from the question of legality, they conflated the two and understood the justness of war in terms of a legal judgment on what constitutes a violation of what the Romans called ius gentium, or law of nations. Chapter 2 focuses on reconstructing the idea of “solemn war” in the works of Grotius, who is seen as the first major thinker to introduce a doctrine of regular war. Indebted to the neo-Roman tradition of war making, Grotius advances that the powers to declare and wage war belong to sovereigns alone, and they must be fought in accordance with “external justice” rather than the “internal justice” of conscience. Chapters 3 and 4 turn to the thought of Wolff and Vattel, who are portrayed as foundational for systematizing regular war in a way that looks recognizably modern. Chapter 5 extends the argument to the nineteenth century and the rise of humanitarianism in order to show the inescapable duality between necessity and humanity.

Admirable in its execution, The Laws of War in International Thought also raises important questions about how to do a critical history of international thought. The first one that deserves special interest concerns the choice of authors and works. By focusing “mostly on some of the ‘great texts’ in legal and political thought,” Kalmanovitz makes a conscious choice to concentrate only on those thinkers who exercised “influence as historical figures” and presented “theoretical sophistication and richness.” Although sensitive to both continuities and discontinuities in any study of an idea that spans more than four centuries, he sees his selected set of authors as “taking part in a common project within the long tradition of natural law and the law of nations” (p. 10). Ideas about war and peace, however, have always been deeply contentious and divisive, and the suggestion that we should see our task as one of uncovering a preexisting body of works called “the laws of war” makes two assumptions that should be questioned. The first is granting the parameters of a field ab initio—or a discipline that purports to be self-defined rather than to be deeply contested and ill-defined, if defined at all. The second, no less warranted, assumption is that differences among authors can be surmountable to justify their belonging to a common project. But to suggest that they should all adhere to and cohere within a recognizable body of thought—called the laws of war—can easily mask some profound differences in their writings, given how sharp and often irreconcilable such divisions can be.

And this brings me to the second, and related, question that concerns the boundaries of the laws of war. Kalmanovitz shows how the evolution of modern laws of war is inextricably linked to customary norms and practices that emerged from “European inter-state warfare” (p. 153). Although the intra-European and inter-European contexts provide the basis for the book, the extra-European dimension is unfittingly absent. Such an absence is all the more puzzling given the historical connection between the laws of war and European expansionism. Their conjunction during the period with which Kalmanovitz is concerned points to the contestable historical origin of the laws of war: the preconditions that enabled their conceptualization and practice occurred precisely during a period defined by an unprecedented rivalry for domination of the world by a mere handful of European nations. It should not be seen as sheer coincidence that all the thinkers examined in the book were themselves—to various degrees—proponents of, if not advocates for, a belief that international law, despite its European origins, should be seen as authoritative globally given Europe’s unique historical position in progress and civilization. Many, if not all, of these thinkers even defended a notion of stadial process, whereby other societies would gradually be admitted to legal equality in a system of “civilized” states, and, either implicitly or explicitly, supported European imperial expansion as a normative project that required domination and inequality.

The connections between empire and international law have only recently been acknowledged in scholarship, along with the gripping tension between law’s aspirations for universal legitimacy and its historically contingent origins in expansion and subjugation. Because of its tangential focus on many of these themes, The Laws of War in International Thought reads more as though it should have been titled The Laws of War in European Thought. What therefore remains to be acknowledged is that, largely until the mid- to late nineteenth century, the law of nations was almost exclusively a European discourse, generated by and produced for the consumption of European empires. It is this reminder that needs to be centered to the core rather than periphery of the evolution of the laws of war.

What Kalmanovitz accomplishes eminently is for all of us to recognize the need to broaden our intellectual horizons when we consider the deeply contested history of the laws of war. His study shows how the triangular entanglement of conscience, necessity, and humanization help frame the uneven discourses and practices that would eventually lead to the establishment of the structures of international governance that—for better or worse—we have inherited in our own present day. The Laws of War in International Thought also serves to remind us how any claim to a singular history (still not all that uncommon in scholarship) obscures the inevitably multilayered evolution of an idea that developed alongside the sovereignty of the states we inhabit today. It is precisely this piece of historical and theoretical development that could also help us not only understand how the contemporary laws of armed conflict came to be but also reimagine the political possibilities of what a world of more “regular war” would mean.