Until Magnifica Humanitas, no pope had stated so plainly that just war theory has run its course. The declaration arrives with particular theological weight: Leo XIV is himself an Augustinian, and it was St. Augustine who, in the fifth century, first gave the Church its framework for thinking about when war could be justified. That a pope formed by Augustinian spirituality should now declare that framework outdated is not a casual remark. It is a considered judgment from within the tradition itself.
What Augustine built
Augustine developed just war theory in response to a specific crisis: how could Christians, committed to peace, participate in the defense of a crumbling empire? His answer established conditions — legitimate authority, just cause, right intention — that the Church refined over centuries through Thomas Aquinas, Francisco de Vitoria, and eventually the Catechism. The framework was never a celebration of war. It was a set of strict limits designed to make war less likely and less brutal.
Why Leo XIV says it is no longer sufficient
Paragraph 192 of the encyclical describes a world in which “historical memory fades and the ethical principles that protect civilians and the most vulnerable are weakened,” making it “easier to justify violence as necessary, inevitable or even sanitized.” The just war framework has become, in practice, a tool for legitimization rather than limitation.
As paragraph 196 notes, entire generations of young people now inhabit a context where war has become “a way of life,” driven by the intertwining of vague ideological motivations with concrete economic interests.
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important to reaffirm that the “just war” theory, which has all too often been used to justify any kind of war, is now outdated. Humanity possesses far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness. The use of force, violence and weapons reflects a relational poverty that always has disastrous consequences for civilian populations. (No. 192).
The specific problem of AI weapons
Paragraph 198 addresses autonomous weapons directly, and without diplomatic softening: “It is not permissible to entrust lethal or otherwise irreversible decisions to artificial systems. No algorithm can make war morally acceptable.” The speed and impersonality that AI brings to armed conflict does not refine the just war calculus but destroys it. “AI does not remove the intrinsic inhumanity of conflict; indeed it can only bring about conflict more quickly and render it more impersonal, lowering the threshold for resorting to violence, transforming defense into threat prediction and thus reducing victims to data.”
What replaces it
Leo XIV is not dismantling Augustine’s moral seriousness. He is insisting it demands more of us now. Paragraph 205 rejects what he calls a “false realism” — “the form of political realism that sows in consciences and in society an attitude of resignation to the inevitability of war.” Peace, he writes, “is neither a naïve hope nor the absence of war; instead, it is always possible as the fruit of justice and charity.”
Paragraph 200 sets out concrete non-negotiables: lethal decisions must remain under “effective, self-aware and responsible human control,” and an international framework must be established to curb the technological arms race.
The Augustinian conclusion
There is a deep coherence in the fact that it is an Augustinian pope who closes the chapter on just war theory. Augustine wrote from within a civilization he could see collapsing. Leo XIV writes from within a different kind of collapse: the erosion of the shared ethical language that made just war theory possible in the first place. His answer, rooted in the Augustinian vision of two cities and two loves that structures the entire encyclical, is that the civilization of love cannot be built with the tools of Babel.
Paragraph 222 puts it with striking directness: “War is never inevitable. Weapons can and must be silenced, for they do not resolve problems but only increase them.”
