Pope Leo, AI, and Just War Theory

Pope Leo XIV’s “Magnifica Humanitas” warns that just war theory has too often been used to justify any kind of war and, in light of modern weapons and artificial intelligence, may be outdated. But is he correct?

From Homer’s “Iliad” to our own constitutional order, civilization has understood that war is never morally self-authenticating. The mere fact that a nation can fight does not mean that it may fight, nor that it may fight however it pleases.

That intuition, disciplined by Christian theology, refined by natural law, translated into international law, and embodied in our Constitution, is called just war theory. Properly understood, it is not a hymn to militarism or a warrant for adventurism. It is a restraint on power, a framework of moral judgment, and a reminder that the sword, even when lawfully borne, remains an instrument to be feared.

The Christian tradition begins with sober anthropology. Man is made in the image of God, but man is fallen. Political authority exists for the common good, yet it is vulnerable to vanity, fear, ideology, and the libido dominandi—the lust for power. The tradition rejects both pacifist abstraction and bellicose enthusiasm. Peace is the normal object of political order, but force may be necessary to defend the innocent, punish grave wrongs, and restore a peace shattered by aggression.

Saint Augustine gave the first systematic Christian account of just war. He insisted that the power to declare war belongs to lawful authority, not factions, mobs, or private militias. He required just cause, describing just war as one that answers grave wrongs, and right intention, admonishing that “peace should be the object of your desire; war should be waged only as a necessity.” Here is the Augustinian genius: Motive matters.

A state may have an army, a grievance, and a strategy, yet still act unjustly if animated by vengeance, cruelty, or domination.

Saint Thomas Aquinas gave Augustine’s insights scholastic precision. In the “Summa Theologica,” he identified three necessary conditions: legitimate authority, just cause, and right intention.

Later thinkers, including Vitoria and Grotius, clarified proportionality, reasonable chance of success, and last resort. The Catechism of the Catholic Church likewise requires grave and certain harm, exhaustion of peaceful means, serious prospects of success, and assurance that the use of arms will not produce evils graver than the evil to be eliminated.

That last condition weighs heavily in an age of nuclear weapons, cyber operations, drones, autonomous systems, and artificial intelligence. The principles endure, but their application must account for technology, scale, speed, collateral effects, and weapons whose consequences earlier centuries could scarcely imagine.

One may respectfully disagree with the pope’s conclusion that just war theory is outdated. But the abuse of a doctrine does not refute it. The remedy for counterfeit currency is not the abolition of money, but the enforcement of standards by which the genuine is distinguished from the fraudulent. Still, the pope’s warning is salutary: If “just war” becomes a benediction placed after policy has already been chosen, the doctrine has been betrayed. Its criteria must precede, constrain, and govern action.

For Americans, legitimate authority is not abstract. Article I gives Congress the power to declare war, raise armies, maintain a navy, and regulate the armed forces. Article II makes the president commander-in-chief. The Framers divided war powers because they understood both the need for executive energy and the danger of executive ambition. A war may be morally defensible yet constitutionally suspect, or lawfully authorized yet morally dubious. Sound statecraft requires both legality and justice.

Just war theory does not end once war begins. The law of armed conflict translates moral principles into operational duties: distinction, proportionality, military necessity, humanity, and honor. Combatants must be distinguished from noncombatants; military objectives from protected objects; necessary force from cruelty. These are not sentimental restraints imposed by people who do not understand war. They are disciplines that preserve the distinction between force and savagery.

History illustrates the point. The Allied war against Nazi Germany remains the paradigmatic modern case of just war: a response to aggression, conquest, genocide, and the destruction of the European order. The Korean War, undertaken after North Korea’s invasion of South Korea, likewise answered aggression and sought to restore an invaded state. By contrast, Nazi Germany’s invasion of Poland, Imperial Japan’s campaigns across Asia and the Pacific, Saddam Hussein’s invasion of Kuwait, and Russia’s invasion of Ukraine were wars of aggression, domination, or annexation.

Artificial intelligence now poses an acute challenge. AI may improve targeting, logistics, defense, and discrimination. Used wisely, it may reduce risk to soldiers and civilians. Used foolishly, it may make war faster, easier, less accountable, and less human. The central principle must be clear: There must be a human in the decision-making loop.

Machines may assist judgment; they must not replace it. Algorithms do not possess conscience, understand mercy, bear guilt, or answer before Congress, a court-martial, a grieving mother, or God.

DoD Directive 3000.09 reflects this truth by requiring appropriate human judgment over autonomous and semi-autonomous weapon systems. The danger is not only that AI will err, but that humans will use AI as a veil for moral abdication: “The system selected the target,” or “the model assessed the risk.” Such statements are evasions. The commander and civilian leader remain responsible.

A just-war framework for AI should therefore require that humans decide whether to use force; approve lethal targeting; test systems under realistic conditions; retain the ability to intervene or abort; and preserve accountability through records, audits, rules of engagement, and legal review. Feasibility is not morality. Speed is not prudence. Precision is not justice.

Just war theory disciplines the statesman, soldier, lawyer, and citizen.

We should reject both absolute pacifism, which cannot defend the innocent, and militarism, which cannot distinguish defense from conquest. The just war tradition stands between them, permitting force when justice requires it but chaining force to authority, cause, intention, necessity, proportionality, discrimination, and humanity.

In an age when weapons are more destructive, targeting more automated, propaganda more powerful, and war more feasible, these principles are not obsolete. They are more necessary than ever.

Ordered liberty depends on structure—constitutional, legal, institutional, and moral. Just war theory is one of the structures by which civilization restrains the violence it sometimes must employ.

We should reaffirm that peace is the object of our desire, and that war, when necessary, must be waged only so that peace may be obtained.