No, the just-war tradition is not ‘outdated’

By Phil Lawler ( bio - articles - email ) | May 26, 2026

Although I generally admired Pope Leo’s first encyclical, I was dismayed to read, in paragraph 192:

Today, more than ever, without prejudice to the right to self-defense in the strictest sense, it is important to reaffirm that the “just war” theory, which has all too often been used to justify any kind of war, is now outdated.

Pope Leo is not the first Pontiff to say that just-war theory is outdated. Pope Francis said the same thing. Even Pope Pius XII, who cannot be classified as a pacifist, said in his Christmas message of 1948: “The theory of war as an apt and proportionate means of solving international conflicts is now out of date.”

But Pius XII, speaking in the aftermath of World War II, did not jettison just-war theory—even with the advent of The Bomb threatening new levels of mass destruction. No one short of Dr. Strangelove would think that nuclear war is “an apt and proportionate means” of resolving conflicts. A just war is always a last resort, when preferable methods have failed.

Pope Leo renders his judgment “without prejudice to the right to self-defense.” But in the just-war tradition, a war can be considered just only if it is fought in self-defense, or in the defense of others.

If a nation can be morally justified in defending itself, how shall we distinguish between which sort of military actions are justified, and which are not? That is precisely the purpose of just-war theory: to make such distinctions. And indeed Pope Leo uses the principles of just-war reasoning when he discusses the dangers of autonomous weapons.

Later in the encyclical, the Pope again speaks inaccurately when he says: “When a culture normalizes and justifies conflict, a dangerous pathway opens up…” Conflict is a reality of human interactions, a consequence of original sin, which no moral system is likely to eliminate. Nations, like individuals, have different wants and needs; sometimes they will be in conflict. The key question, international affairs, is how those conflicts will be resolved, and of course peaceful negotiations are the preferred method.

Clausewitz famously characterized war as “the continuation of diplomacy with other means.” Pope Leo is certainly right to prefer means of resolving diplomatic conflicts short of war. But again, if all other means fail, and if the consequences of an unjust peace are even more serious than the horrors of war, the Church has traditionally taught that military action can be warranted—and in some cases may be required. Surely the Holy Father, as a loyal son of St. Augustine, must understand that great saint’s argument that the only purpose of a just war is to secure a just peace.

Now if Pope Leo had suggested that just-war theory needs to be updated, I could wholeheartedly agree. In Magnifica Humanitas he outlines some of the reasons why the just-war tradition must be brought into line with 21st-century realities.

In discussing the danger of autonomous weapons—military systems guided by artificial intelligence, without immediate human oversight—the Pontiff invokes the principles of just-war reasoning when he says that “moral judgment cannot be reduced to calculation, for it involves conscience, personal responsibility and the recognition of the other as a person.” He goes on, in the next paragraph (#199) to give a more extended example of just-war analysis:

It is not enough to invoke a generic type of ethics. Concrete criteria for discernment must be established. The first such criterion concerns personal responsibility. When a decision to strike becomes automated or opaque, the risk of abdicating responsibility increases. For this reason, the chain of responsibility must be identifiable and verifiable; those who design, train, authorize and employ technology must be held accountable for their decisions. The second criterion pertains to the moral timeframe for making judgments. While AI tends to expedite the decision-making processes, speed and efficiency should never be the supreme motivating force for the irreversible decisions made in the context of war. The third criterion is the identification and protection of civilians. Any technology that facilitates attacks without seeing the face of human beings lowers the moral threshold of conflict. Target selection and the use of force must not confuse combatants and non-combatants, nor ignore the impact on defenseless populations.

Earlier in the encyclical the Pope mentioned other, non-military forms of aggression that might be used, in the manner that Clausewitz envisioned, to force a diplomatic adversary into submission: “Alongside conventional warfare, there are hybrid forms such as cyberattacks, information manipulation, campaigns of influence and the automation of strategic decisions.” To give just one example, the use of an EMP (electromagnetic-pulse) weapon, crippling the energy grid, could cause more devastation and ultimately more deaths than a bombing campaign. Closer to the theme of the papal encyclical, disinformation campaigns—made far more powerful by AI capabilities—can wreak havoc on the democratic process. These are forms of aggression, tantamount to warfare. A cyberattack of sufficient scale might even be considered a classic example of a casus belli.

These developments, which the Pope mentions in his encyclical, deserve more attention from just-war moralists. One might add the need for a deeper discussion of pre-emptive war, in an age when hesitation could mean national destruction. And the increasingly prominent role played by non-state actors—such as terrorist groups, often acting with barely-concealed support from governments—merits more moral discernment as well. How can a nation, acting justly, respond to these new dangers? The challenge for Christian moralists today is to update, not jettison, the just-war tradition.