It isn’t every day that a pope calls for an overhaul of a more than 1,000-year-old teaching of the Catholic Church, but that’s exactly what Pope Leo XIV did last month. In his inaugural encyclical, “Magnifica Humanitas,” which was mainly an exploration of how to protect human dignity in the age of artificial intelligence, Leo devoted a brief but critical passage to just war theory.
In a break with a foundational principle of Catholic thought on conflict, Leo called the theory “outdated” and made it clear that the teaching has been twisted to justify wars for decades, most recently the war in Iran. It is about time for the change.
Just war theory holds that wars must meet strict conditions: They should be in self-defense, and only if alternatives have been exhausted; the use of force should be proportional; there should be a likelihood of success and the threat should be imminent. Since World War II at least, several popes have criticized world leaders for using the theory as a fig leaf.
While Leo did not cite any specific war in the encyclical, he clearly had President Trump’s war on Iran in mind. On June 6, in remarks en route to Madrid for a visit, he was asked if a “just war” was being waged in Iran. The pontiff replied: “I believe this has already been made very clear: In Iran, the criteria for a just war are not present.” Leo wasn’t done. “The theory of the just war dates back to centuries when it was impossible to imagine the weapons and the destructive capacity available to humanity today,” he added.
Just war theory was largely developed by St. Augustine in the 5th century and elaborated by St. Thomas Aquinas in the 13th century. For an institution that is averse to change, the idea that anyone, even a pope, would rewrite the teaching of a revered saint might seem to some to be unorthodox, even radical. Yet who better suited to overhaul a major teaching of St. Augustine than this pope, a former head of the religious order named for Augustine, who was taught by Augustinians starting as a teenager and wrote his dissertation on the order’s governance?
In casting doubt on the usefulness of just war teaching in the modern era, Leo’s encyclical preserved “the right to self-defense in the strictest sense.” Instead of suggesting a new framework for justifying war, Leo all but rules out war’s legitimacy. “Humanity possesses far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness,” Leo wrote. “The use of force, violence and weapons reflects a relational poverty that always has disastrous consequences for civilian populations.”
Other recent popes have also seen a recurring problem in the distortion of just war theory. In “Fratelli Tutti,” Pope Francis said in 2020 that every war in recent decades had been “ostensibly ‘justified,’” even when they weren’t just. “War can easily be chosen by invoking all sorts of allegedly humanitarian, defensive or precautionary excuses, and even resorting to the manipulation of information,” he said.
Leo has gone further, making the case that the age of artificial intelligence undermines the moral criteria for just war.
Only humans are capable of making moral judgments, he argued. Autonomous weapons systems make war “more ‘feasible’ and less subject to human control,” Leo wrote. “This violates the principle that armed force should be used only as a last resort in cases of legitimate self-defense.” Machines should not be making decisions about war, Leo said.
Leo’s episcopal allies in the United States have made clear that the pope was thinking about Iran in the months before the encyclical’s release, on May 25. On April 10, Leo posted on his official X account, “God does not bless any conflict. Anyone who is a disciple of Christ, the Prince of Peace, is never on the side of those who once wielded the sword and today drop bombs.”
In response, Vice President JD Vance, who is Catholic, cited just war theory at a college campus speech. “When the pope says that God is never on the side of people who wield the sword, there is more than a 1,000-year tradition of just war theory. We can, of course, have disagreements about whether this or that conflict is just.” (Mr. Vance, at a later commencement speech at the U.S. Air Force Academy, stepped back from his criticism and praised the encyclical and the call for an update of just war principles given new technologies in warfare.)
Central to the church’s critique of the Trump administration’s approach to the war in Iran is that it has offered contradictory, murky and mercurial justifications for the conflict. Some of the administration’s arguments included elements of just war principles, such as the idea of an imminent nuclear threat from Iran. But Leo and his American allies argue there is almost no justification for war without exhausting dialogue, diplomacy and forgiveness. War can be morally legitimate only in self-defense or in extreme circumstances to repel evil in the world.
Overhauling just war teaching will be among the top issues to be discussed on Friday and Saturday at a consistory, or meeting of the pope’s cardinals, at the Vatican, according to Vatican News, the Holy See’s official outlet. The cardinals should not pull their punches. The moment urgently calls for new guidance, not just discussion. In reformulating the church’s view of war, the stakes for Catholicism, the United States and the world are high, and the Vatican needs to get this right. The universal church, with millenniums of moral reasoning and clergy on the ground in virtually every conflict zone, is uniquely situated to articulate a new intellectual framework on just war theory — especially as A.I. increasingly automates decisions on the battlefield.
The answer is deceptively simple. Leo, his papal predecessors and his contemporary brethren are calling for returning to Christian roots, with one simple concept: War is fundamentally antithetical to the Gospel of Jesus Christ.
James V. Grimaldi, a Pulitzer Prize winner, was an executive editor of The National Catholic Reporter, where he directed coverage of the conclave that elected Pope Leo XIV.
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