When Augustine of Hippo wrote The City of God in the early fifth century, Rome was collapsing around him. He lamented the horrors of war, yet he also recognized that governments bear responsibility for preserving order and restraining evil. Augustine argued that just wars arise because of the wrongdoing of aggressors and that political authorities sometimes have a duty to protect the innocent when peaceful remedies fail.
From that hard recognition emerged the Christian just-war tradition: not a license to fight, but a moral framework designed to make war harder, not easier, to justify. Sixteen centuries later, Pope Leo XIV has declared it obsolete. In paragraph 192 of Magnifica Humanitas, his encyclical released May 25, Pope Leo writes that just-war theory “which has all too often been used to justify any kind of war, is now outdated.” He argues that humanity now possesses “far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness.”
Every Christian can honor that desire for peace. The encyclical’s conclusion on this point, however, rests on a misunderstanding of what the just-war tradition teaches — and it arrives at exactly the wrong moment, when artificial intelligence is remaking warfare at a pace no diplomatic instrument can match.
What the Just-War Tradition Was Actually Designed to Do
Just-war doctrine was never a theological permission slip for ambitious princes. When Thomas Aquinas codified Augustine’s reasoning into formal criteria in the thirteenth century, every element was conceived as a restraint on power. Legitimate authority prevents private actors from waging war on personal grievance. Just cause limits conflict to resisting genuine aggression. Right intention rules out conquest and vengeance as acceptable aims. Last resort requires that statesmen genuinely pursue
peaceful remedies before reaching for the sword. Proportionality forbids using more force than the threat demands. Discrimination protects civilians from deliberate targeting.
Each criterion was designed to make going to war morally harder, not easier. The doctrine has been abused across centuries — Leo is right about that — but the answer to the abuse of a sound principle is to apply it more rigorously, not to abandon it. We do not discard contract law because contracts are sometimes breached.
History vindicates the doctrine when leaders follow it. The Allied response to Nazi Germany met every just-war criterion: aggression was undeniable, diplomacy had been exhausted at Munich, and military resistance became morally necessary to halt a catastrophic evil. The 1991 Gulf War coalition rested on the same grounds — an aggressor had violated international borders, peaceful remedies had been genuinely pursued, and coalition forces acted with proportionate force to restore the status quo. History’s condemnation falls not on Augustine’s framework but on those leaders who chose to ignore it.
The ongoing conflict with Iran offers a more searching test. The United States and Israel launched Operation Epic Fury on February 28, 2026, targeting Iran’s nuclear infrastructure and military leadership. Christians across denominations have invoked every just-war criterion to evaluate those strikes — questioning whether last resort was truly satisfied when Omani mediators reported a diplomatic framework still within reach, whether a president acting without new congressional authorization met the standard of legitimate authority, and whether proportionality was observed given the civilian casualties that followed. Those are exactly the right questions to ask. That they are being asked — vigorously, publicly, across the Church — proves the doctrine is functioning as Augustine intended: as a moral check on the temptation to use force. Remove the framework, and there is no vocabulary left with which to hold a government accountable. The answer to a contested war is not to abolish the criteria. It is to apply them with greater discipline.
The present makes the same case. Russian forces entered Ukraine in February 2022 and have continued to shell civilian infrastructure, occupy sovereign territory, and forcibly deport Ukrainian civilians. Ukrainian resistance satisfies the right Pope Leo himself acknowledges, self-defense “in the strictest sense.” The just-war criteria are not making that resistance harder to justify — they are the only internationally legible moral framework by which Ukraine’s defense can be distinguished from Russia’s invasion, and on which the moral and material support sustaining Ukraine depends. The doctrine is not the obstacle to peace — the aggression is.
Where Leo Is Right—and Why It Points Back to the Tradition
Pope Leo is at his most persuasive when Magnifica Humanitas turns to autonomous weapons. He warns that any technology facilitating attacks “without seeing the face of human beings lowers the moral threshold of conflict,” and insists that decisions involving life and death “must not be entrusted to machines.” As a retired U.S. Army infantry officer who has written extensively on these questions in The New AI Cold War, I take that warning seriously. The battlefield of the near future involves autonomous drone swarms, AI-assisted targeting, predictive intelligence networks, and cyber weapons operating at machine speed. The Department of War’s DoD Directive 3000.09, Autonomy in Weapon Systems, updated in January 2023, requires that commanders retain “appropriate levels of human judgment over the use of force” precisely because machines making lethal decisions without human oversight is a live danger, not a hypothetical one.
Seen clearly, every danger Leo identifies in AI-enabled warfare is an argument for applying just-war doctrine more rigorously, not for retiring it. Artificial intelligence compresses decision cycles and lowers the threshold for initiating conflict — which is precisely why last resort becomes more indispensable, not less. Autonomous systems distribute and obscure accountability across command-and-control chains, which is why legitimate authority becomes a sharper requirement than ever. Machine-speed targeting raises the risk of uncontrolled escalation, demanding more careful attention to proportionality. Targeting algorithms that cannot reliably distinguish combatants from civilians make the principle of discrimination more urgent, not obsolete. Augustine’s framework has not been overtaken by technology. It has vindicated it.
Leo’s diagnosis of the AI age’s dangers is sound. Where the encyclical goes astray is in concluding that those dangers discredit the tradition rather than calling it back into force. Scripture’s teaching in Genesis 1:27 that human beings bear the image of God is the theological foundation on which just-war reasoning rests. A machine carries no such image and bears no moral guilt. When an autonomous system misidentifies a civilian target, no algorithm faces a court-martial, and no targeting model confronts its conscience before God. That is not an argument for abandoning moral frameworks around warfare — it is the most powerful argument available for insisting that human beings, commanded in Romans 13 to bear the sword as God’s servants for good, must never surrender that accountability to a machine. The theological case for just war has seldom been more urgent than it is right now.
The Problem Has Always Been Disobedience to the Doctrine
The Pope’s proposed alternatives — dialogue, diplomacy, and forgiveness — are not actually alternatives to just-war doctrine; they are already embedded within it as requirements. Last resort has always been one of the tradition’s core requirements. The framework demands that peaceful options be genuinely pursued before force is ever considered, and that war be undertaken to restore peace rather than achieving conquest. Far from competing with diplomacy, just-war doctrine elevates it by making recourse to arms morally difficult to justify. What no doctrine can do is substitute for diplomacy once diplomacy has already failed — which is precisely the situation Augustine was addressing, and precisely the situation that confronts the world today.
Pope Leo XIV has done something important. By devoting a major teaching document to artificial intelligence and warfare, he has forced a global conversation that Christian statesmen, military planners, and pastors have largely avoided. His warning that decisions involving life and death must remain in human hands, not in algorithms, deserves to be taken seriously across every faith group. That much of the encyclical stands.
Where the document falls short is in urging the retirement of a moral framework rather than its more disciplined application. The future battlefield will be shaped by lethal drones, AI-assisted command systems, and autonomous platforms operating at speeds that compress human decision-making toward the vanishing point. The questions that will matter most in that environment are the same ones Augustine posed in the ruins of Rome — who authorized the use of force, were peaceful alternatives genuinely exhausted, were the innocent protected — and no algorithm will ever be equipped to answer them. As I develop in AI for Mankind’s Future, the Church’s task in the age of artificial intelligence is not to retire the frameworks that discipline warfare but to insist, with renewed urgency, that they govern it. The human being created in God’s image — not the machine built in a laboratory — must remain the moral center of every decision about lethal force.
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