Pope Leo questions just war theory in first encyclical on AI

Pope Leo XIV used his first encyclical, Magnifica Humanitas, to challenge a pillar of Catholic moral teaching as artificial intelligence reshapes modern warfare. The 82-page document, published May 15, 2026, calls for “the most rigorous ethical constraints” on AI in war and argues that just war theory has too often been used to justify almost any conflict.

The encyclical places war and peace inside a larger meditation on human dignity in the age of AI, while also addressing marriage and family, economic and social life, and the political community. Leo described just war theory as “outdated,” except in cases of “self-defense in the strictest sense,” a sharp rebuke to a tradition that has long shaped Catholic thinking on when force can be morally justified.

That tradition is usually traced to St. Augustine and later developed by St. Thomas Aquinas, but the Catholic Church’s own Catechism sets narrow conditions for using force. It says the use of arms must not produce evils graver than the evil to be eliminated, and that the destructive power of modern weapons must weigh heavily in judging whether war can be justified. Leo’s language, paired with his warning about autonomous weapons, pushes that standard into the age of AI-driven targeting and machine speed decision-making.

The reaction has already split along familiar lines. On April 15, 2026, Bishop James Massa, chairman of the U.S. Conference of Catholic Bishops’ Committee on Doctrine, said the church has taught just war theory for more than a thousand years and that a war can be just only in self-defense after peace efforts fail. Cardinal Blase Cupich, a close ally of Leo, said the pope is concerned about how world leaders have used the theory to justify war and that dialogue, diplomacy and forgiveness offer better tools.

The dispute reaches beyond Catholic theology. In a country already wrestling with drone warfare, autonomous systems and the moral limits of force, Leo’s encyclical turns a doctrine once used to restrain violence into a test of whether the church will keep treating modern war as morally governable at all.