Pope Leo and President Trump: Convergence, Divergence and Evolution of Just War Theory and International Law

[Nicolás Zambrana Tévar LLM (LSE) PhD (Navarra) is Associate Professor at KIMEP University School of Law]

In recent weeks, a remarkable public dispute has captured global attention. Pope Leo XIV—the first American-born pontiff, a Chicago-born Augustinian friar named Robert Prevost—and President Donald Trump have exchanged unusually sharp statements over the United States and Israel’s war against Iran. The Pope called Trump’s April 7 threat to destroy “a whole civilization” “truly unacceptable.” Trump responded by calling Leo “WEAK on Crime” and “terrible for Foreign Policy.” Cardinal Robert McElroy of Washington, D.C., went further: “In Catholic teaching, this is not a just war,” he declared. “This is a war of choice.” 

Why does this exchange matter beyond the usual political noise? The media attention may partly reflect an eagerness to highlight any challenge to the Trump presidency, whatever its source. But something more significant is happening. The Pope and the President are re-enacting a debate that is nearly two millennia old: under what conditions, if any, may a nation wage war? That debate produced one of the most relevant intellectual inheritances of Western civilization—the doctrine of just war—which forms in part the deep moral and legal grammar of modern international law. But that grammar is now under serious strain, from theologians who argue that modern warfare has made just war theory practically obsolete, even as international lawyers still operate within its inherited categories.

From Augustine to Aquinas: The Architecture of Just War

The intellectual genealogy of just war theory begins with Saint Augustin of Hippo in the early fifth century. In The City of God and Contra Faustum, Augustine argued that war could be morally justified when it aimed at restoring peace and was undertaken with the right intention—justice being a property of cause, intent, and authority, not of victory. Augustine drew from Cicero’s political philosophy and the Roman jus fetiale, a body of sacred law administered by a priestly college that determined whether the gods sanctioned a proposed war. The jus fetiale recognized four just causes: unauthorized incursion into Roman territory; infractions of the rights of ambassadors; violations of treaties; and military assistance given to an enemy—and only after a formal demand for satisfaction had been rejected. This Roman framework—requiring a prior wrong, a legitimate authorizing institution, and a proportionate aim—flowed into Augustine’s Christian framework, then into Aquinas, and ultimately into the architecture of modern international law.

The Middle Ages transformed Augustine’s framework into a practical system of restraint. The Peace and Truce of God (tenth through twelfth centuries) imposed canonical protections on non-combatants and restricted the times at which violence could occur. Pope Innocent II, at the Second Lateran Council of 1139, attempted to prohibit the crossbow as excessively lethal. Rulers who violated these norms faced excommunication; Henry IV was famously brought to his knees at Canossa after Gregory VII excommunicated him.

Thomas Aquinas gave just war theory its canonical scholastic formulation in the Summa Theologiae (II–II, q. 40): legitimate authority (auctoritas principis), just cause (causa iusta), and right intention (intentio recta). Later scholastics added proportionality and last resort as additional requirements. Like Augustine, Aquinas is explicit that victory is irrelevant to moral evaluation—war may be unjust even when won, and just even when lost.

The Spanish Scholastics and the Birth of International Law

The conquest of the Americas forced European theologians to confront the limits of their tradition. The Dominican Francisco de Vitoria and the Jesuit Francisco Suárez asked whether Spain could justifiably wage war against indigenous peoples who had committed no wrong against it. Vitoria’s answer was essentially no: titles based on religion, papal donation, or civilizational superiority were insufficient. This revolutionary insight laid the foundations of modern international law. Hugo Grotius, in De Iure Belli Ac Pacis (1625), explicitly acknowledged his debt to Vitoria and the School of Salamanca. The key move was secularization: the same moral criteria—just cause, proportionality, legitimate authority—were detached from their theological foundations and reframed as rules of natural law applicable to all sovereign states.

After Westphalia: War as a Legal Institution

The Peace of Westphalia (1648) accelerated this secularization. War was treated less as a morally justified exception and more as a legitimate instrument of state policy. The doctrine of raison d’état displaced the scholastic demand for a just cause. As Carl Schmitt later argued, the classical European system achieved a kind of moderation precisely by refusing to moralize war: both sides were treated as legitimate belligerents rather than criminals.

Here lies a paradox essential to understanding the Trump–Leo dispute: in the centuries after Westphalia, when Europe was still nominally Christian, international law was least restrictive of war. The prohibitions that look most “Christian”—the UN Charter’s near-absolute ban on the use of force—emerged in a less explicitly theological era, driven by two catastrophic World Wars, nuclear weapons, and the Kantian insight anticipated in Perpetual Peace (1795), that only a legal international order could prevent civilizational destruction.

The United Nations Charter as Secularized Just War

The Charter of the United Nations (1945) is history’s most ambitious inadvertent attempt to juridify just war theory. Article 2(4) establishes a near-absolute prohibition on the use of force; Article 51 preserves an “inherent right of individual or collective self-defence”; and the Security Council may authorize collective force to respond to threats to international peace. The structural parallels with scholastic doctrine are striking: self-defense maps onto just cause; the Security Council reproduces the requirement of legitimate authority; proportionality and necessity are now legal standards under the Geneva Conventions and International Humanitarian Law.

Yet as the legal theorist Martti Koskenniemi has shown in his landmark From Apology to Utopia, translating moral criteria into legal standards produces characteristic instability. What counts as an “armed attack”? What is “proportionate” force? Is preemptive war against a nuclear-armed state legitimate under Article 51? These questions—urgently relevant to both Iraq in 2003 and Iran today—require precisely the moral and prudential judgment that just war theory was designed to provide.

Catholic Teaching in the Twentieth Century: From Regulation to Skepticism

Throughout the twentieth century, popes progressively narrowed the moral space for justified war, moving well beyond the framework of the UN Charter and, arguably, beyond even formal Catholic doctrine as codified in the Catechism of the Catholic Church (1992, sections 2302–2317). The Second Vatican Council’s Gaudium et Spes (1965, §80) declares that “any act of war aimed indiscriminately at the destruction of entire cities or of extensive areas along with their populations is a crime against God and man himself” —effectively ruling out nuclear warfare in terms which can be described as more emphatic than the Additional Protocol I to the Geneva Conventions of 1977. John Paul II opposed both the Gulf War (calling it “an adventure with no return”) and the 2003 Iraq War, writing in Centesimus Annus (1991, §52): “No, never again war.” While being interviewed in 2003, before being elected Pope, Benedict XVI asked whether, “given the new weapons, it is still licit to admit the very existence of a ‘just war’.” Pope Francis, in Fratelli Tutti (2020, §258), stated that “it is very difficult nowadays to invoke the rational criteria elaborated in earlier centuries to speak of the possibility of a ‘just war’”—stopping short of formal abolition, since the Catechism still contains the doctrine, but rendering it nearly inapplicable in practice.

Pope Leo XIV has continued this trajectory. He denounced the “delusion of omnipotence” driving global conflicts, warned that “God does not bless any conflict,” and wrote that a disciple of Christ “is never on the side of those who once wielded the sword and today drop bombs.” When Trump responded that “God is good and God wants to see people taken care of”—implicitly invoking divine endorsement for the Iran war—the theological stakes became unmistakable.

The Deeper Shift: From Casuistry to Conscience

This evolution of the Catholic approach to the morality of war reflects a broader transformation in moral theology which, before Vatican Council II, was predominantly manualist—moral questions treated as the application of fixed rules to specific cases. Catholic theology has since shifted toward a more personalist and historically conscious approach, emphasizing conscience, human dignity, and the recognition that judgments are made in concrete, evolving contexts. As Benedict XVI argued in his 2005 Address to the Roman Curia, authentic doctrinal development preserves continuity of principle while allowing genuine historical transformation in its formulation. This is the so called “‘hermeneutic of reform’, of renewal in continuity.” Applied to just war, the shift is decisive: the classical model presupposed that criteria could yield determinate conclusions; the modern sensibility emphasizes the epistemic difficulty of applying them under conditions of modern warfare—technological complexity, asymmetric actors, mass civilian casualties, and unpredictable escalation.

Cardinal McElroy’s statement illustrates this precisely: “You cannot satisfy the just war tradition’s criterion of right intention if you do not have a clear intention.” The problem is not that the criteria are wrong, but that they cannot be meaningfully applied when the war’s objectives—eliminating Iran’s nuclear capacity, regime change, unconditional surrender—are themselves unclear and shifting.

Secular Voices: A Convergent Tradition

The Catholic trajectory is not isolated. Erasmus argued in Querela Pacis (1517) that war is almost always morally absurd. Kant’s Perpetual Peace proposed legal abolition through a federation of republican states. The Kellogg–Briand Pact (1928) was the first formal attempt to renounce war as an instrument of national policy. Contemporary revisionist just war theorists such as Jeff McMahan have concluded that most real wars fail even minimal justification criteria—a position sometimes called “contingent pacifism.” The Catholic and secular philosophical trajectories thus converge not necessarily because of direct influence, but because they are responding to the same structural reality: modern technology has made just war criteria—designed for armies with swords and crossbows—functionally inapplicable to aerial bombardment and nuclear warheads.

The Three-Level Divergence and its Stakes

We can now identify the central paradox that the Trump–Leo exchange dramatizes, operating at three levels simultaneously. Classical Catholic theology affirms that war is sometimes justifiable, and the Catechism still codifies the conditions. Modern international law prohibits war in principle while permitting structured exceptions—self-defense, Security Council authorization—that reproduce the scholastic framework in legal form. But contemporary Catholic moral theology, expressed through the trajectory from John Paul II to Leo, holds that these exceptions are practically unreachable under modern conditions. The paradox: the secular legal order, which emerged from the secularization of just war theory, remains more permissive toward war than the theological tradition that gave birth to it.

This divergence has direct implications for the Iran war debate. The Trump administration argues that the campaign meets the criteria of self-defense: Iran’s nuclear ambitions and state-sponsored terrorism constitute just cause, and the President has legitimate authority. Pope Leo and Cardinal McElroy are not simply disagreeing about facts; they are challenging the framework itself—insisting that “right intention” requires clear and morally coherent objectives, that “proportionality” demands consideration of mass civilian suffering, and that “last resort” means genuinely exhausting diplomacy. These are just war criteria applied with a severity that renders the bar far higher than positive international law requires.

The Continuing Influence of Papal Teaching

Historically, the Church exercised its restraining role through institutional mechanisms: canon law, the threat of excommunication, diplomatic pressure on Catholic monarchs. Today, it operates through the moral and pastoral authority of the papacy in a media-saturated public sphere. When Pope Leo says “military action will not create space for freedom or times of peace”, he is exercising moral suasion that potentially reaches 1.4 billion Catholics worldwide, shaping the normative climate in which governments must justify their decisions. The fact that JD Vance, a Catholic convert, felt the need to publicly tell the Pope to “stick to matters of morality” is telling. Archbishop Paul Coakley stated plainly: “Pope Leo is not his rival, nor is the pope a politician. He is the vicar of Christ, who speaks from the truth of the Gospel and for the care of souls.”

Conclusion: No Just War Anymore?

Are recent Popes right that there is no just war anymore? Formally, no: the Catechism still articulates the conditions, and international law still recognizes self-defense and Security Council-authorized force. Practically, the trajectory of Catholic moral teaching strongly suggests the bar has risen so high as to be nearly unreachable in modern conditions—a genuine moral evolution rooted in the shift from casuistry to historically conscious prudential judgment, and in the recognition that modern warfare systematically violates the conditions just war theory requires. Structurally, international law and contemporary Catholic theology are increasingly out of alignment: law continues to regulate war as a manageable exception; theology increasingly doubts whether it can be justly managed at all. The Trump–Leo dispute is, at its deepest level, a dispute between these two frameworks—one that the classical just war tradition, in its long and fertile history, never quite anticipated.

The first Augustinian pope and the president who claims divine endorsement for war are, whether they know it or not, re-enacting a debate that has defined Western civilization’s struggle to limit violence for sixteen centuries. That debate is far from over.

Photo attribution: Photo by Xavier Coiffic on Unsplash