The Catholic Church has not abandoned the right to self-defence, but it has increasingly rejected the idea that modern war can be morally justified.

The category of the “just war” – born with Augustine, systematised by Aquinas – was, for centuries, the Christian conscience’s attempt to subject violence to moral criteria. The magisterium of the recent popes has progressively eroded its ground, down to Leo XIV’s explicit move beyond it.

The turn begins in the atomic era. Pius XII restricted the licit recourse to arms to defence alone. As early as 1944, in his Christmas radio message, he had declared “now out of date” the theory of war as “an apt and proportionate means of solving international conflicts”. It was “out of date” in Pius XII and “outdated” in Leo XIV: the same word nearly 80 years apart.

John XXIII, in Pacem in Terris (1963), declared it alienum a ratione – foreign to reason – to imagine that, in the nuclear age, war could serve as an instrument of........

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