Pope Leo this week repudiated a major teaching used by the Catholic Church since at least the fifth century to evaluate when countries might be justified in waging wars, in a move experts said could have long-reaching impact for global powers.

The disavowal of the doctrine came in the pope’s first major document, issued on Monday, which also urged global regulation of AI systems and made the clearest apology yet for the Catholic Church’s historic role in supporting transatlantic slavery.

“The ‘just war’ theory which has all too often been used to justify any kind of war, is now outdated,” wrote Leo in the encyclical, entitled “Magnifica Humanitas” (Magnificent Humanity).

“Humanity possesses far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness,” he said.

Chicago Cardinal Blase Cupich, a close ally of Leo who was at the Vatican for the presentation of the text on Monday, told Reuters the pope is concerned with how the theory has been used by world leaders to justify going to 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,” said Cupich.

‘Just War’ invoked by US VP Vance

Leo, who has adopted a more forceful tone in recent months and has drawn the ire of US President Donald Trump after criticising 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 that wars should only be waged in order 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 mentioned the just war theory at an event in the state of Georgia and urge 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 (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.

Theory used in military academies

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 the year 430, proposed specific criteria to evaluate whether a war could be considered just. He said wars ⁠should only be waged with the intention of restoring a state of 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 US.

They have been also invoked by some critics of the Iran war to argue that the conflict, started by ​surprise US-Israeli airstrikes against Iran on 28 February, 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 US, who see hope in the proven effectiveness of nonviolent strategies for protecting democracy, transforming conflict, and legitimate defense,” she said.