Just war thinking and the law of war constitute intersecting, interwoven conversations that often reflect each other like mimes in a mirror: just war thinkers cite legal arguments to defend ethical intuitions while lawyers turn to ethics and philosophy to work around the strictures of the law. This extensive overlap is not surprising, given the histories of these two traditions. In the West, just war thinking emerged in the fourth century as a theological and political response to a fundamental problem of world order. Threatened both internally and externally, the newly Christianized Roman Empire had to confront the question of whether or not force could permissibly be used for political ends. But just over a millennium later, just war thinking had virtually disappeared from Western political discourse.

As just war scholarship faded from the scene, Protestant scholars adapted its logics to create a new language of discourse: international law. In the process, a theological tradition once focused on the health of individual souls was transformed into a legal tradition focused on the rights of states. The international legal tradition developed rapidly in the late nineteenth century, and the devastating world wars of the twentieth century spurred the creation of the thick web of laws and institutions that now govern the practice of war. Unlike the Christian just war tradition that preceded it, today's international law thinks of itself as truly universal—not just in its application but also in its supposed reflection of universal norms. Furthermore, international humanitarian law has come to focus on protecting individuals’ rights in war, a move that further distances it from its roots in the historical just war tradition. These developments in the legal approach to the ethics of war have in turn influenced contemporary just war thinking, which has been enjoying a robust secular revival since the publication of Michael Walzer's Just and Unjust Wars in 1977. In other words, we have come full circle, with law now influencing the ethical approach.

Given this sea change in the way we think about war, is just war thinking still relevant? If just war thinking was meant to serve as a moral compass in a lawless and anarchic international system, what can it tell us today, in an international system characterized by increasingly dense networks of laws and institutions? Do we need a parallel moral conversation? What are the points at which just war thinking and the law should intersect? Should contemporary just war thinkers be informing the interpretation of the law? And should their ethical positions reflect recent developments in the law?

The authors in this roundtable offer a wide range of responses to these questions, from three distinct disciplines: religion, political science, and law. David Luban argues that international law serves a practical function by imposing clear definitions and principles on the murky world of international conflict and by providing a set of “off the shelf” rules for war. Although just war thinkers might argue that in some cases legal decisions are considerably off the moral mark, Luban points out that there are good reasons to accept the law of war, imperfect as it may be. Similarly, Valerie Morkevičius suggests that just war thinking and international law serve different, but equally valuable functions. While the law works externally to motivate us primarily through shame, just war thinking works internally to motivate us through guilt. Because of guilt's motivating power, just war thinking can move beyond the law in several ways, from providing a method for addressing moral injury as a society to identifying ethical problems in radically new contexts.

Both James Turner Johnson and Edward Barrett make the case that one of just war thinking's most useful contributions is its ability to identify and respond to new ethical challenges. Johnson argues that a better understanding of the roots of just war thinking—and not just the Western ones—can help address contemporary challenges to the existing world order posed by nonstate actors, and even reinforce respect for international law. In the same vein, Barrett draws on the example of cyber warfare to warn that the attempt to impose old legal frameworks on new problems can lead to less, rather than more, clarity. He argues that in this case not only would the law greatly benefit from engaging more deeply with just war thinking but that just war thinking can equally benefit from engaging with the law. Together, the authors agree that there is lasting value in both just war thinking and international law.

Despite the considerable overlap between the two traditions, it is their differences that make conversations within and between them so valuable. Rather than evangelization, such conversations provide an opportunity for scholars of each tradition to share insights and to challenge unexamined assumptions, deepening and strengthening our understanding of how wars are and should be fought.