Just War Theory after Nuclear Weapons

or those of us with a bit of seniority, looking back on the immediate aftermath of the Cold War, George H. W. Bush’s promise of a “global spring” and Francis Fukuyama’s prediction of the “end of history” now seem remarkably optimistic, if not naïve. Depending on how one counts, more than one hundred armed conflicts are taking place around the world today, at least eight of which qualify as major wars involving large-scale combatant and civilian casualties. According to the Uppsala Conflict Data Program, armed conflict claimed approximately five million lives between 1989 and the end of 2025. Even that sobering estimate may be conservative. Depressingly, as the old song reminds us, the beat goes on.

Against this backdrop, the need for moral clarity in thinking about war is no less urgent today than at any point in history. For centuries, the just war tradition has provided the Western moral tradition with an indispensable framework for evaluating both the justice of resorting to war (jus ad bellum) and the justice of conduct in war (jus in bello). Yet every major advance in military technology has tested the tradition’s core principles—especially proportionality, discrimination, and just cause. In earlier centuries, those innovations included the crossbow, firearms, and aerial bombardment. More recently, and with arguably greater consequence, nuclear weapons and artificial intelligence have led some observers to conclude that just war thinking has become obsolete.

The most influential contemporary voice advancing this argument is Pope Leo XIV. In his inaugural encyclical, Magnifica Humanitas (2026), he declared that “just war theory, which has all too often been used to justify every kind of war, is now outdated.” In doing so, Pope Leo went beyond his predecessor, Pope Francis. Although Francis declared that “the possession of nuclear weapons is immoral,” he stopped short of rejecting the just war tradition itself. Leo’s central claim is that “the just war theory comes from centuries past when we couldn’t imagine the weapons [and] human beings’ ability for destruction.”

Leaving aside the fact that conventional bombs and bullets killed as many as eighty-five million combatants and noncombatants during the Second World War, there is little doubt that nuclear weapons represent something sui generis in the history of warfare. Robert Oppenheimer captured this reality after witnessing the Trinity test when, quoting the Bhagavad Gita, he reflected, “Now I am become Death, the destroyer of worlds.”

After 1945, a diverse group of secular and religious thinkers—including Bertrand Russell, Hannah Arendt, Herman Kahn, Reinhold Niebuhr, and Paul Ramsey—recognized the unprecedented strategic and moral implications of nuclear weapons. Within the Roman Catholic Church, the principal institutional guardian of the just war tradition, these developments prompted a sustained reevaluation of the Church’s teaching on war and peace.

That reassessment began with Pope John XXIII’s 1963 encyclical Pacem in Terris. Because the encyclical questioned whether war could still serve as an instrument of justice in the atomic age, some interpreted it as inaugurating a new pacifist ethic in which no war could be morally justified. Yet commentators from very different theological perspectives—including Michael Novak and J. Bryan Hehir—rejected that reading. In retrospect, Pacem in Terris did not represent a rupture with the Church’s historic teaching on war and peace. Rather, it reflected a growing awareness that the nuclear age required a more searching application of traditional moral principles.

The Church’s second major postwar reassessment occurred during the Second Vatican Council (1962–1965), particularly in Gaudium et Spes. Like Pacem in Terris, the council’s treatment of war and peace proved both influential and controversial. Vatican II continued the Church’s movement away from viewing warfare as an acceptable instrument of policy in the nuclear age, while nevertheless reaffirming the Church’s millennia-long commitment to the just war tradition.

Following Vatican II, skepticism toward nuclear weapons continued to deepen, particularly within the American Catholic Church. By the mid-1970s, the U.S. bishops had concluded that any use of nuclear weapons was morally impermissible while permitting their possession, under strict conditions, as a lesser evil in support of deterrence—a position later affirmed by Pope John Paul II in his 1982 address to the United Nations. This moral formula—use, no; possession, qualified yes—set the stage for the National Conference of Catholic Bishops’ landmark 1983 pastoral letter, The Challenge of Peace: God’s Promise and Our Response. Arguably no ecclesial document has more comprehensively examined the biblical, moral, and strategic dimensions of war and peace in the nuclear age.

The Challenge of Peace represented the culmination of two decades of Catholic reflection that began with Pacem in Terris. It forcefully condemned the use of nuclear weapons while allowing, as a strictly transitional measure, a policy of nuclear deterrence. At the same time, it reaffirmed the just war tradition, emphasizing its strong presumption against the use of force. The pastoral letter also advocated arms control, encouraged respect for conscientious objection, and gave renewed prominence to nonviolence and peacebuilding within Catholic moral reflection.

The pastoral letter generated national headlines, and the responses it provoked reflected the theological and political divisions of the era. Cardinal John J. O’Connor welcomed its central affirmation that peace remained possible in the nuclear age. George Weigel, while applauding its reaffirmation of the just war tradition, criticized its treatment of deterrence and what he regarded as an insufficient appreciation of strategic realities. Reagan biographer William Inboden notes that both President Ronald Reagan and his administration followed the drafting of the pastoral letter closely, viewing it as a potential challenge to the administration’s strategy of peace through strength in its confrontation with the Soviet Union.

The legacy of The Challenge of Peace has been profound, shaping subsequent Catholic teaching while influencing evangelical, mainline Protestant, and even secular discussions of war and peace. Pope Leo XIV’s Magnifica Humanitas represents the latest stage in this trajectory. Yet the encyclical is arguably more than an aggiornamento—an updating—of Catholic teaching. It is the strongest papal statement to date that modern warfare, especially in an age of nuclear weapons, hypersonic missiles, autonomous systems, and artificial intelligence, has rendered the traditional conditions for a just war increasingly difficult to satisfy.

Inspired by Pope Leo’s encyclical, some have gone a step further, concluding that the time has come to lay the just war tradition to rest. Others, however, while acknowledging the profound challenges he identifies, argue more persuasively that our task is not to abandon this venerable tradition but to reaffirm its enduring relevance in the face of radical technological change.

*Dean C. Curry lives in the Lowcountry of South Carolina and has spent many years teaching and writing on subjects related to international relations, foreign policy, and religion and public life.

Source: https://providencemag.com/2026/08/just-war-theory-after-nuclear-weapons/