Ethics Before Deployment: Catholic Just War Theory and Doctrinal Development for Military AI

On 25 May 2026, Pope Leo XIV's Magnifica Humanitas became the first papal encyclical on artificial intelligence, declaring that lethal decisions cannot be entrusted to artificial systems and calling for a shared international framework to govern military AI. It states these principles but does not operationalise them. This article develops that operational framework. Applying Catholic social doctrine (human dignity and agency, subsidiarity, prudence, intentionality, the common good, and the protection of noncombatants) to military AI, it translates the encyclical's commitments into concrete requirements for autonomous weapons, targeting systems, and alliance interoperability. It grounds the argument in the nuclear precedent, where Catholic ethics shaped doctrine as it formed, and more briefly in chemical and biological weapons prohibition. Engaging just war theory, NATO's Principles of Responsible Use, recent military wargaming, and a leading AI provider's treatment of a catastrophic strike as consistent with its self-declared “red lines,” it shows where Catholic ethics is more demanding than procedural compliance. It then proposes institutional mechanisms: a tiered deliberation framework, proportionality dossiers, ethics review boards, and NATO-level coordination. Building ethical architecture before deployment, the article argues, serves strategic as well as moral interests.