When Pope Leo took heat from Donald Trump and the MAGA movement for denouncing the war in Iran earlier this year, much of his Church stood with him. Conservative and liberal Catholics alike argued that Leo was right to condemn the fighting, given the administration’s holy-war rhetoric and its inconsistent justifications of the conflict.

This broad agreement among Catholics, however, hid a more fundamental dispute—not about Trump or Iran, but about the principles that guide how the Church thinks about war. The “just war” theory, which draws from Saint Augustine’s writings during the fall of the Roman empire and was systematized in the Middle Ages by Saint Thomas Aquinas, exists to address a persistent theme in human history: Bad actors regularly commit terrible deeds, and nations must decide when and how to defend themselves, or to intervene on behalf of others.

According to the framework, an armed conflict must have a just cause, such as responding to an unprovoked attack. It must be pursued as a last resort and have reasonable hope of success. It must be motivated by good intentions and carried out by a legitimate authority. It must rely only on just means, such as using proportional force and distinguishing between combatants and civilians. Its expected good must outweigh its inevitable harm.

Despite its long lineage, just-war theory’s influence in Catholic thought has been dwindling for decades as conflicts have become more complex and weapons more destructive. To some observers, the clearest sign yet of the framework’s fall from favor came in May, when Pope Leo wrote in his encyclical that it is “outdated.”A month later, 178 cardinals met at the Vatican to discuss whether the Church should move away from the language of just war. Still, many Catholics support the theory and dispute the suggestion that it’s passé.

They believe that it was, and remains, the best way the Church has to make sense of war. At the center of the debate is a deeper question: How can the Church help limit violence in an all-too-violent world, in which nuclear bombs, terrorist attacks, and AI-guided drones are ever-present threats?

Just-war theory isn’t just the stuff of papal encyclicals. Military academies teach courses and host conferences about it. Human-rights laws are in part derived from it. Prominent pundits and politicians have opposed and defended some of the largest conflicts of the past few decades on just-war grounds.

Since the mid-20th century, though, the Vatican has seemed more focused on promoting peace than on explaining the ins and outs of the theory. The Church has tended to emphasize nonviolence “as the first resort and oftentimes an effective alternative to the use of military force,” Gerard Powers, who directs the Catholic Peacebuilding Studies program at the University of Notre Dame, told me. Pope John XXIII wrote about the need for nuclear disarmament; Pope Paul VI made an impassioned anti-war plea at the United Nations; Saint John Paul II described war as “always a defeat for humanity.”

Recent Catholic scholarship, moreover, has highlighted the Church’s role as a mediator in conflicts—between the Colombian government and the Revolutionary Armed Forces of Colombia, for example—and criticized just-war theory for being too focused on the ethics of war rather than on nonviolent ways of preventing it.

Popes have also voiced doubts about how applicable just-war theory can be today. In 2003, the future Pope Benedict XVI said that “we should be asking ourselves if it is still licit to admit the very existence of a ‘just war.’”In 2020, Pope Francis wrote that “it is very difficult nowadays” to “speak of the possibility of a ‘just war.’”As critics of the theory have pointed out, modern weapons and tactics have far more destructive power than they were in the late Roman and medieval period, when just-war theory was formulated, which makes just-war principles especially difficult to apply. When a military uses aerial bombing, civilians are almost certain to be among the casualties. Autonomous weapons raise new questions about moral accountability, which is the core of any ethical theory.

Some critics also assert that just-war theory has been “ineffective or inadequate at preventing and restraining war,” as Eli McCarthy, a lecturer in theology at Georgetown University, told me. “Even if we see a war that looks like it might meet the just-war criteria,” he said, those same wars tend to “get us stuck in cycles of violence.”The United States entered World War II, for example, to respond to Japan’s attack on Pearl Harbor, to protect Europe against fascism, and to end the Holocaust.

That conflict also led to the Cold War and several proxy wars. America invaded Afghanistan after the 9/11 terrorist attacks killed thousands of civilians, but that war did not permanently rid the country of the Taliban, and escalated the War on Terror.

The Vatican’s zeal for peace and seeming ambivalence about just-war theory has drawn criticism, especially among a conservative contingent of American Catholics. In the early 2000s, the philosopher Michael Novak and the writer George Weigel contended that the Church’s reflexive opposition to war downplayed governments’ moral obligation to uphold order and keep their citizens safe from autocrats and terrorists. Weigel wrote that the post–World War II Church had adopted a “functional pacifism” that ignored centuries of just-war teaching.

Novak suggested that the Church underestimated the asymmetries presented by terrorism: Ideologically motivated, non-state actors intent on destruction at all costs are unlikely to be moved by diplomacy and may well be stopped only by force. More recently, J. D. Vance admonished Pope Leo in April “to be careful when he talks about matters of theology,” after the pontiff’s criticism of the Iran conflict. “There is more than a 1,000-year tradition of just-war theory,” Vance said.

To skeptics of just-war theory, these criticisms exemplify its biggest flaw: how susceptible it is to being interpreted too broadly, or co-opted for political goals. Novak and Weigel defended the invasion of Iraq on just-war grounds, as did Catholic politicians including Joe Biden and the late U.S. Representative Henry Hyde.

Just-war theory’s defenders have answers to this objection and others. John Schwenkler, a philosopher at the University of Texas at Austin, told me that the fact that the theory is “only as good as those whose hands it is in” doesn’t invalidate it. Just-war theory, he said, was meant to be “a framework within which to reason and debate,” not a “decision manual” that provides ready-made answers. If anything, the sloppy use of just-war theory makes clarifying its criteria “more important than ever.”

Theorists have also applied just-war criteria to new technologies. After World War II, when popular opinion held that the use of atomic bombs in Hiroshima and Nagasaki had been a “necessary evil,” the Oxford University philosopher Elizabeth Anscombe argued against their use on just-war grounds. She and others maintained that the scale of destruction, though ghastly, was secondary: What mattered was that the United States had intentionally targeted civilians—and that was unjust. To the theory’s supporters, examples such as this show that it is neither overly permissive nor outdated; instead, it sets a high bar for the legitimate use of force and can be used to think about the ethics of novel technologies.

In the early Middle Ages, when soldiers returned from battle, the Catholic Church would at times impose spiritual penalties called “penances” on them for injuring or killing someone. The particulars varied—being denied Holy Communion for a time, or being told to eat only bread and water on certain days. “I think it’s actually quite a profound thing,” Phil Klay, a Catholic writer and Iraq War veteran, told me about the practice. “You’d have to seek forgiveness even if you were in a just war.”

This tension is at the root of the Church’s debates about just-war theory. Many supporters and critics agree that the theory is prone to abuse. They also agree that peace is precious. But their disagreement underlies how all wars, no matter how righteous, are tragic because they require killing people who, as Klay put it, are “made in the image of God.”

For now, the Catholic Church seems to be distancing itself from just-war theory even more overtly than in the past. At the September conference in Rome, Cardinal Charles Maung Bo, the archbishop of Yangon in Myanmar, told attendees that “we need a new paradigm” that recognizes nonviolence as “a more effective form of defence.”Cardinal McElroy said that Leo’s deemed that the theory “can no longer constitute an authentic pathway of moral guidance in the life of the Church.”But earlier this year he also said that “there are times when military force is morally legitimate.”

Most Catholics invested in the Church’s teaching on war understand that despots, fanatics, and the human urge to dominate can make war unavoidable. For all of its faults, just-war theory recognizes this. Any new Church teaching would need to contend with this reality too.