Pope Leo this week upended a long-standing Catholic teaching used since at least the fifth century to assess when war may be considered justified, a move experts say could have far-reaching implications for global powers.
The shift came in his first major document, issued Monday, which also called for global regulation of artificial intelligence and included what is seen as the church’s clearest apology yet for its historic support of transatlantic slavery.
In the encyclical, titled Magnifica Humanitas (Magnificent Humanity), Leo wrote: “The ‘just war’ theory, which has all too often been used to justify any kind of war, is now outdated.”
He added that humanity now has more effective tools for resolving conflict, including dialogue, diplomacy and forgiveness.
Chicago Cardinal Blase Cupich, a close ally of Leo who attended the Vatican presentation of the document Monday, told Reuters the pope is concerned about how the doctrine has been used by world leaders to justify war.
“We have to make clear that the just war theory was always meant to be a restraint, not a permission slip, which sadly some are misusing to justify their decisions to go to war rather than seek the ways of peace,” Cupich said.
Leo, who has adopted a more forceful tone in recent months and has drawn the ire of U.S. President Donald Trump after criticizing the Iran war, decried the number of wars roiling the world in his text and warned that arms industry profits were a driving force behind conflicts.
The just war theory, which generally says wars should be waged only to defend against aggression, has been invoked by Trump administration officials, including Vice President JD Vance, a Catholic, to defend the Iran war.
In April, after the pope’s official X account posted that God “is never on the side of those who once wielded the sword,” Vance cited the just war theory at an event in Georgia and urged the pope “to be careful when he talks about matters of theology.”
Anna Rowlands, a British academic who was part of Monday’s Vatican presentation of the pope’s document, told Reuters that Leo is expressing concern about “a new age of changing conflict, now increasingly tech-driven.”
“It is a strong statement about the need for [the just war theory] to be placed in a renewed, wider context of criteria for building peace and resolving conflict,” she said of the pope’s declaration that the theory is outdated.
The just war theory was first articulated by St. Augustine of Hippo, a major figure of the early church who Leo has said inspired him to become a priest. The pope is a member of the Augustinian religious order, founded on the saint’s teachings.
Augustine, who died in 430, proposed criteria to evaluate whether a war could be considered just. He said wars should be waged only with the intention of restoring peace and never out of a desire for cruelty.
His criteria remain a cornerstone of curricula at military academies across the world, including at West Point, the Naval Academy and the Air Force Academy in the United States.
They have also been invoked by some critics of the Iran war to argue the conflict, started by surprise U.S.-Israeli airstrikes against Iran on Feb. 28, is unjust.
Washington Cardinal Robert McElroy, for example, said in April that the war was “morally illegitimate,” citing Augustine’s principles.
Marie Dennis, a former leader of the international Catholic peace movement Pax Christi, said Leo’s document “exposes the fiction of a ‘just war’ with the truth about a culture of power that is normalizing war.”
“Pope Leo joins millions of others around the world, including in the United States, who see hope in the proven effectiveness of nonviolent strategies for protecting democracy, transforming conflict and legitimate defense,” she said.
