As the modern state system evolved during the centuries before the seminal event of the Peace of Westphalia in 1648, international law gradually displaced just war doctrine in providing guidelines for permissible uses of international force. During this process, international law absorbed the basic just war guidelines, building a bridge of continuity between the religious outlook of medieval Europe and the emergent secularism of the modern state system. Just war thinking, which is rooted in the religious traditions of the West, especially Christianity, is usually grounded in the fifth century writings of the great Catholic theologian, St. Augustine. In contrast, international law has freed itself from direct religious authority in stages. By the nineteenth century, international law was tied to the consent of sovereign states, an expression of secularism, and the legacy of the rationalism and scientific ethos associated with the triumph of the Enlightenment in the West.
This legalist approach to war, intervention, and force reached its climax in the period immediately after World War II. The Nuremberg Tribunal punished surviving Nazi leaders for crimes against peace, and the UN Charter made an unconditional prohibition on the use of non-defensive force its core commitment. Nuremberg also punished Crimes of War and Crimes Against Humanity; namely acute legal violations associated with conduct as distinct from recourse to force, paralleling the just war distinction between jus ad bello and jus in bello. International law became an entirely autonomous source of authority with respect to the use of force and the exclusive guideline for statecraft, although theologians and moralists on the sidelines continued to debate limits on force.
How, then, can we explain this just war revival in discussions about the use of force since the end of the Cold War, and especially in the period after September 11, 2001? The essential explanation lies in the revolutionary changes in the technology of warfare, the nature of conflict, and the doctrine of territorial sovereignty. These changes burst the bounds of international law. They also propelled the search for ways to think about force that avoided the extremes of sterile legalism and political nihilism. International law subsequently lost much of its legitimacy as an acceptable source of guidance for the leaders of liberal democracies, which were the only states that were somewhat inclined to orient their behavior by reference to these legal constraints. Even here, it is easy to exaggerate. US leaders after World War I, partly under the dual influences of US President Woodrow Wilson and anti-war public opinion, seemed to endorse a law-oriented approach to issues of war and peace. However, this was severely attacked by such prominent realists as George Kennan and Hans Morgenthau as having paved the way to World War II, emboldening aggressor nations while inducing the democracies to fall into a condition of geopolitical slumber. Realism prevailed in the period of the cold war, and it was only in civil society that international law became a focus for opposition to "illegal" uses of force. This dynamic reached its peak during the latter years of the Vietnam War.
But then why did the just war tradition reemerge? It reemerged because international law no longer seemed sufficient, and at the very least needed to be reinforced by ethical constraints, as an instrument of persuasion in civil society. The limits imposed by international law seemed to be unable to accommodate the development of nuclear weapons. The question was whether a goal of war can justify relying on such apocalyptic weaponry. International law, caught between its endorsement of wars based on defensive necessity and its vague rejection of excessive and indiscriminate force in accordance to the law of war, was helpless to respond. The most important formulation on these issues from the just war tradition was a widely influential 1983 Pastoral Letter by the Catholic Bishops of North America addressing the question of the role of nuclear weaponry in the Cold War context of mutual deterrence. …