Thomas Aquinas on the Morality of War

Last updated on: April 8, 2026 at 9:01 am by Barbara O'Brien

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Thomas Aquinas on the Morality of War 2026-04-08T09:01:26-05:00 Barbara O'Brien

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Saint Thomas Aquinas (Tommaso d’Aquino, 1224/25-1275) developed precepts about the morality of war that influence international law to this day. Now the saint’s writings on war are relevant to current events, as Christian views on war have been in the news,

Recently U.S. Secretary of Defense Pete Hegseth led a prayer service in the Pentagon that drew what appeared to be a rebuke from Pope Leo XIV. Invoking the war in Iran, the Secretary prayed for every round to find its mark, and for “overwhelming violence of action against those who deserve no mercy.” The prayer concluded, “We ask these things with bold confidence in the mighty and powerful name of Jesus Christ.”

A few days later, Pope Leo spoke to the incompatibility of Christianity with the glorification of war. In his Palm Sunday homily, the pontiff said, “Brothers and sisters, this is our God: Jesus, King of Peace, who rejects war, whom no one can use to justify war. He does not listen to the prayers of those who wage war, but rejects them, saying: ‘Even though you make many prayers, I will not listen: your hands are full of blood.'”

On Easter Sunday President Trump expressed confidence that God is on his side in the Iran conflict. I’m not sure Saint Thomas Aquinas would agree. Many consider Thomas Aquinas to be Catholicism’s greatest theologian. Let’s consider what he had to say about war.

Saint Thomas Aquinas on War

Saint Thomas Aquinas was one of the giants of western philosophy and theology. Aquinas was born in Sicily, where his parents owned a modest feudal domain. When Tommaso was five years old his parents placed him in a nearby Benedictine monastery where an uncle was abbot. A clash between Emperor Frederick II and Pope Gregory IX caused the monastery to close in 1239. Young Tommaso was then sent to a studium generale — a medieval university — in Naples. It’s believed he began his formal study of philosophy there. He also chose to join the Dominican order. The Dominicans must have recognized his intellectual gifts and sent him to the University of Paris to continue his education. He would spend much of the rest of his life in Paris, producing masterpieces of theology.

One of those masterpieces was the Summa Theologiae, a compendium of the theological teachings of the Church. And in a section of the Summa Aquinas wrote about war. You can read that section online here. Is war always sinful? he asked. Can war be just? Is killing in war the same as murder? In answering these questions, Aquinas explored many perspectives and often seemed to be arguing with himself. Aquinas consulted the Gospels and many earlier philosophers. You’ll notice that Aquinas quoted Saint Augustine of Hippo (354-430) in several places. Here is an earlier column on Saint Augustine’s just war theory that addresses many of the same issues.

To judge the morality of going to war, Aquinas said, circumstances and intentions must be considered carefully. Especially intentions. War is sinful, he said. But sometimes it is unavoidable. War is just and lawful when it is conducted under three conditions: First, it must be declared by a sovereign power, the head of government. Second, one’s cause must be just. If an adversary attacks and plunders other countries, that adversary “should be attacked because they deserve it ” Needless to say, attacking other countries to seize their territory or wealth is not a just cause. And third, a just war requires rightful intentions, the intentions to do good and not evil. Here Aquinas quotes Augustine — “The passion for inflicting harm, the cruel thirst for vengeance, an unpacific and relentless spirit, the fever of revolt, the lust of power, and such like things, all these are rightly condemned in war.” Any selfish intention, including a desire for personal aggrandizement, would be condemned.

“Those who wage just wars intend peace,” Aquinas wrote.

A Note on Intentions

Several centuries and a whole lot of history have passed since Saint Thomas Aquinas wrote the Summa Theologiae. And if you study that history, you realize nearly every group of people who chose to go to war persuaded themselves that they were justified, that they were the “good guys,” and that God was on their side. Nearly every atrocity in human history began with the thought, “I’m a good person,” followed by “his hatred of me is evil, but my hatred of him is justified.” As soon as we identify ourselves as “good” and the Other, whoever they are, as “evil,” we’re well on the way to giving ourselves a cosmic permission slip to do whatever we want to be rid of them. People are seduced into evil because they don’t recognize evil as evil. They mistake it for justice, or righteousness, or even God’s Will. Intentions must be infused with humility and a willingness to consider that maybe God is not on our side. I’m not seeing a lot of that in Washington right now.

Gunpowder was introduced to Europe during Aquinas’s lifetime, but cannons weren’t used in a European war until about 60 years after Aquinas died. War was mostly hand-to-hand combat in his day. He dedicated a section of his comments on war to a discussion of whether it is lawful to use ambushes and other subterfuge in war. This seems almost quaint. Aquinas also addressed fighting on holy days: “[I]t is lawful to carry on a war on holy days, provided there be need for doing so: because it would be to tempt God, if notwithstanding such a need, one were to choose to refrain from fighting. However, as soon as the need ceases, it is no longer lawful to fight on a holy day.” I take that to mean if people must fight to defend themselves on a holy day, that is permissible. Otherwise, no.

Aquinas also discouraged clergy from taking up arms, even in a just cause. “Prelates ought to withstand not only the wolf who brings spiritual death upon the flock, but also the pillager and the oppressor who work bodily harm; not, however, by having recourse themselves to material arms, but by means of spiritual weapons, according to the saying of the Apostle (2 Corinthians 10:4): ‘The weapons of our warfare are not carnal, but mighty through God.’ Such are salutary warnings, devout prayers, and, for those who are obstinate, the sentence of excommunication.”

Trained glass portrait of Saint Thomas Aquinas. Source: Wikimedia Commons, CC BY-SA 2.0 license

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About Barbara O'Brien

Barbara is the author of The Circle of the Way: A Concise History of Zen from the Buddha to the Modern World (Shambhala, 2019). You can read more about the author here.

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