Pope Leo XIV arrives to celebrate the funeral service for late Cardinal Camillo Ruini, in St. Peter's Basilica at the Vatican last month.Andrew Medichini/The Associated Press
The first American pontiff has not been shy about taking on the 47th American president.
A year into his papacy, Pope Leo XIV has emerged as a powerful critic of Donald Trump’s “inhuman” crackdown on migrants and questioned his no-holds-barred approach toward the development of artificial intelligence.
Yet, no papal reproach has piqued Mr. Trump’s ire more than the Pope’s criticism of his decision to launch a war on Iran. After Leo weighed in on the conflict in April, saying Christ “is never on the side of those who once wielded the sword and today drop bombs,” Mr. Trump posted an AI-generated meme depicting himself as a Jesus-like figure. That was just after he called the Chicago-born pontiff “weak on crime” and “terrible on foreign policy.”
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Days later, Vice-President JD Vance, a convert to Catholicism, provided his own riposte to Leo’s anti-war comments, albeit in a more diplomatic tone. “How do you say that God is never on the side of those who wield the sword?” Mr. Vance asked, pointing to the liberation of France from the Nazis. “[T]here is a more than 1,000-year tradition of just war theory.”
Indeed, for centuries, the Catholic Church has deemed that military violence can be supported in certain circumstances, provided it is conducted in self-defence, is a last resort, has a serious prospect of success and does not produce harms greater than those which it seeks to stop. Over time, the criteria set out by the Church’s just war theory came to be used by Western political leaders to defend military action. Mr. Vance is just the latest.
Pope Leo has not just signalled his disapproval of the use of this doctrine to provide moral cover for the U.S.-led attacks on Iran; he appears to be seeking to scrap the just war theory altogether. This would be no minor development for an institution that moves slowly. The just war doctrine originated in the 5th century with the writings of St. Augustine.
In May, Leo gave a clear indication that he intends to use his pontificate to replace the just war doctrine when he published his first encyclical, entitled “Magnifica Humanitas.” While the bulk of the document was devoted to setting out the moral principles that should guide the creation and use of AI, it included a chapter specifically devoted to the influence of modern technology on the way and ease with which war is now waged.
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“In our time, a culture of power is taking hold, in which the availability of resources and the ability to dominate tend to dictate the agenda and criteria for decision-making,” Leo writes. “This culture of power infiltrates society, changes relationships and behaviours, and grows by normalizing war, pursuing ever-greater military power, taking advantage of the crisis of multilateralism and fuelling a false realism that insists that there is no alternative.”
The Pope baldly states: “Today, more than ever, without prejudice to the right to self-defence in the strictest sense, it is important to reaffirm that the ‘just war’ theory is outdated.”
Last week, Leo gathered 178 cardinals at the Vatican to discuss the encyclical in what is known as a consistory, with a revision of the just war theory on the agenda. Cardinal Victor Manuel Fernández, head of the Vatican’s Dicastery of the Doctrine of the Faith, led the discussions, warning that the theory was being “manipulated to provide a theoretical foundation for the most unjust wars.”
Cardinals stand on a balcony after U.S. Cardinal Robert Prevost was elected Pope Leo XIV by the conclave last year.Hannah McKay/Reuters
The cardinals weighed in on whether to replace the just war theory with the much narrower concept of “proportional defence.” In his closing remarks to the consistory, Leo said any shift must “be further developed with the necessary theological and pastoral rigour.”
Still, for a Church that thinks in terms of millennia, the American pope appears impatient to change this one doctrine. The urgency stems, in part, from the Vatican’s concerns over the U.S. military’s use of Palantir software that relies on Anthropic’s Claude AI model to identify targets in Iran. Though the Pentagon has insisted that all strikes are ultimately approved by human beings, the use of AI has vastly expanded the number of potential targets and the scope of destruction.
Leo’s encyclical warns that “the growing ease with which autonomous weapons systems can be deployed makes war more ‘feasible’ and less subject to human control.” It adds that “moral judgement cannot be reduced to a calculation, for it involves conscience, personal responsibility and the recognition of the other as a person.”
In seeking to rewrite the just war theory, Leo would not just remove a convenient crutch for waging military violence. He might – one can only hope – even change the course of history.