Justifying war

Governments have been justifying war as long as there have been governments. Egyptian pharaohs asserted a monopoly on legitimate violence and the right to employ it against challengers. They often claimed divine authority for their campaigns. “I went north because I was strong enough to attack the Asiatics through the command of Amon, the just of counsels,” said Kamose in the fifteenth century BC.

Aristotle justified wars in self-defense. “The proper object of practising military training is not in order that men may enslave those who do not deserve slavery, but in order that first they may themselves avoid becoming enslaved to others.”

In the Christian era, Augustine of Hippo elaborated on just wars. Augustine followed the pharaohs in stating that a just war must accord with divine will. “When war is undertaken in obedience to God, who would rebuke, or humble, or crush the pride of man, it must be allowed to be a righteous war,” he wrote.

Augustine echoed Aristotle in authorizing wars of self-defense. “Soldiers should perform their military duties in behalf of the peace and safety of the community,” he said.

Augustine declared that just wars must be fought in righteous spirit and not from base motives. “The real evils in war are love of violence, revengeful cruelty, fierce and implacable enmity, wild resistance, and the lust of power,” he said.

Augustine’s strictures framed Christian reflections on war for a millennium. Thomas Aquinas codified them in the 13th century. “In order for a war to be just, three things are necessary,” Aquinas wrote in Summa Theologica. “First, the authority of the sovereign by whose command the war is to be waged.” Private individuals must take their disputes to court.

“Secondly, a just cause is required, namely that those who are attacked should be attacked because they deserve it on account of some fault.” The appropriate purpose of war was to punish wrongdoing.

“Thirdly, it is necessary that the belligerents should have a rightful intention, so that they intend the advancement of good or the avoidance of evil.” A bad attitude could spoil even a worthy cause.

Until Christendom broke apart in the Protestant Reformation of the sixteenth century, the war thinking of Augustine and Aquinas carried real weight. Popes wielded the sanction of hellfire over violators. Innocent III excommunicated the army of the Fourth Crusade after it diverted from fighting Muslim infidels to sacking Christian cities on behalf of Venetian creditors.

But after Martin Luther and Henry VIII survived excommunication without obvious harm, papal threats lost some of their terror. Just war theory became secularized again, culminating in conventions and protocols promulgated by international bodies in the twentieth century. The Geneva conventions constrained the conduct of war, especially regarding civilians and prisoners. The charter of the United Nations forbade war except in self-defense or in accord with resolutions of the Security Council to maintain or restore peace.

Through all this, it was hard to tell whether the various pronouncements on just war had any meaningful effect. War didn’t disappear. The wars of the twentieth century were the worst in human history. The absence, as of 2026, of a third world war appeared to have more to do with the fear of nuclear destruction than with a consensus that such a war would be morally wrong.

Sometimes it seemed that the principal function of just war theory was to provide advocates of war with the language for rationalizing it. Nearly every war was described as a matter of self-defense. Germany asserted self-defense in invading Belgium in 1914 and Poland in 1939. Japan did the same in attacking Pearl Harbor in 1941. America pleaded self-defense in Korea in 1950, Vietnam in the 1960s, Afghanistan and Iraq in the 2000s and Iran in 2026.

Enough! said Pope Leo XIV in May 2026. After acknowledging the non-occurrence of a third world war, the first American pontiff lamented the proliferation of smaller wars. “We are witnessing a real paradigm shift in public discourse and in decisions regarding rearmament, with a troubling revival of war as an instrument of international politics, while the very ethical principles that had previously limited its use are being eroded,” Leo wrote in the encyclical Magnifica Humanitas. “In our time, a culture of power is taking hold, in which the availability of resources and the ability to dominate tend to dictate the agenda and criteria for decision-making. In this way, the common good of humanity is relegated to the background and the concrete tragedy of peoples at war is reduced to a secondary consideration in relation to strategic interests.” War was being seen as inevitable. “This culture of power infiltrates society, changes relationships and behaviors, and grows by normalizing war, pursuing ever-greater military power, taking advantage of the crisis of multilateralism and fueling a false realism that insists that there is no alternative.”

Social media erased the memory of war and selectively reported on its current conduct, Leo said. The advent of artificial intelligence threatened to make war automatic, beyond the immediate control of national leaders.

The time had come to reconsider the whole concept of just war. “Peace no longer appears as a responsibility to be taken on, but as a fragile interval between conflicts,” Leo said. “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.”

Leo left the self-defense loophole, but he wanted it to become a last resort. “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.”

Was the world listening? It had often ignored Augustine and Aquinas or perverted their words. Would it heed Leo?