The evolution of Just War Doctrine from Pius XII to Leo XIV

 THE JUST WAR AND THE RECENT POPES

The Catholic Church now completely rejects “just war” theory, declaring all armed conflict outdated in favor of disarmed diplomacy.

Antonio Spadaro SJ

AI-generated illustration depicting six recent popes above St. Peter’s Basilica in Vatican City.

The category of the “just war”—born with Augustine, systematized 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 defense alone. As early as 1944, in his Christmas radio message, in fact, he had declared “now out of date” the theory of war as “an apt and proportionate means of solving international conflicts.” It was “out of date” in Pius XII and “outdated” in Leo XIV: the same word nearly eighty years apart.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 justice.

The Council, in “Gaudium et Spes,” called for war to be evaluated “with an entirely new attitude” and condemned total war as a crime against God and against man himself.

John Paul II carried this line down into history: his “never again war” rang out against the Gulf War and, later, against the intervention in Iraq, while “Centesimus Annus” had already called war a “self-destruction of mankind.”

Benedict XVI, more sober in tone, had staked out the position while still a cardinal: asked about the war in Iraq (in an interview with Zenit, on May 2, 2003), he observed that modern weapons strike far beyond combatants, and that therefore “today we should be asking ourselves if it is still licit to admit the very existence of a ‘just war.”

With Francis, the doubt becomes a verdict. In “Fratelli Tutti” (No. 258), he writes that it is “very difficult” to invoke rational criteria elaborated in other centuries to speak of a possible “just war,” and he closes with an imperative: “Never again war!”

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From “Pacem in Terris” to “Magnifica Humanitas,” the Church has never ceased to recognize the right to self-defense; it has ceased to lend war the comfort of an adjective.

In a footnote, he adds that Augustine “forged a concept of ‘just war’ that we no longer uphold.”

This is not merely the diagnosis of a “world war fought piecemeal”; it is doctrinal development: at Hiroshima, in 2019, he declared immoral not only the use but the very possession of nuclear weapons.

And to Patriarch Kirill, in March of 2022, with Ukraine under invasion, he said that one can no longer speak, as was once done, of “holy war” or “just war.”

Leo XIV takes a further step. In the encyclical “Magnifica Humanitas,” he denounces a “paradigm shift”: the rehabilitation of war as an ordinary instrument of international politics, even as the ethical principles that once limited its use erode.

In this context, he writes, “without prejudice to the right to legitimate defense in the strictest sense,” that the theory of the just war, “too often used to justify any kind of war,” is “now outdated.”  Humanity has more effective instruments at its disposal: dialogue, diplomacy, forgiveness.

And he extends the argument to the newest frontier—artificial intelligence applied to weapons, which removes the lethal decision from the reach of conscience. His programmatic formula, proclaimed from the loggia on the day of his election, is a peace that is “disarmed and disarming.”

Not a rupture but maturation: Tradition deepening its self-understanding. From “Pacem in Terris” to “Magnifica Humanitas,” the Church has never ceased to recognize the right to self-defense; it has ceased to lend war the comfort of an adjective.