Saint Augustine’s Just War

Last updated on: March 12, 2026 at 2:01 pm by Barbara O'Brien

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Saint Augustine’s Just War 2026-03-12T14:01:03-05:00 Barbara O'Brien

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What is a “just war”? Saint Augustine of Hippo (354-430 CE) is credited with formulating a just war theory that remains embedded in western philosophy. Augustine drew upon Christian teachings as well as Greek and Roman philosophy to propose a set of principles that defined a just versus an unjust war. Generations of theologians and philosophers have refined and expanded Augustine’s ideas about war. Today much of Augustine’s work is reflected in the international humanitarian law that were codified in the Geneva Conventions after World War II.

Augustine of Hippo was a Christian theologian and philosopher. He was born in Africa, in what is now Algeria, which then was part of the Roman Empire. He was well educated, especially in classical Latin literature. As a young man he explored diverse spiritual and philosophical paths. He also was known to be a hedonist. He wrote in his Confessions that he had “a cut and bleeding soul” and could find no healing — “Not in the company of a dinner table, not in the delights of the bed: not even in my books and poetry.” Eventually he found what he was seeking in Christianity and converted in 386 at the age of 31. He was ordained a priest in 391. He became Bishop of Hippo (today’s Annaba, Algeria) in 395.

Augustine was one of the giants of western civilization. His written works profoundly shaped the development of western Christianity and had a deep impact on western philosophy. His writings on the nature of the Trinity, original sin, free will, the nature of evil, and the integration of faith and reason shaped western Christianity for more than a millennium and are still closely studied by serious theologians. For more on Augustine’s contributions to western thought, I suggest The Enduring Influence of St. Augustine in Western Philosophy by Anthony R. Etuk and the entry on Augustine of Hippo in the online Stanford Encyclopedia of Philosophy. Here I’m just going to provide a very basic introduction to Augustine’s ideas on just war. And note that I am not presenting Augustine’s ideas because I agree with all of them — some I do, some I don’t — but I think they’re interesting and useful to understand.

Augustine’s Just War: Historical Context

For some time before Augustine, most Christians of the Roman Empire had embraced pacifism. In the interpretations of many early Church fathers, Jesus taught absolute nonviolence. Christians were sometimes executed — and became martyrs — for refusing to fight for Rome. The priest Hippolytus of Rome (170-235) refused to baptize soldiers. Early Christian communities within the Roman Empire saw themselves as separate from Roman civic life. Especially when the emperors were pagan, it seemed wrong to give allegiance to them. But then came Constantine I, the first Christian emperor of Rome, who ruled from 306 to 337. This change caused many Christians to re-think their relationship with their country and emperor. Augustine’s thoughts on just war were written to persuade pacifist Christians that a soldier can fill a moral and Christian purpose.

Among the first to address the question of Christian pacifism versus duty to country was Ambrose, Bishop of Milan (339-397). Ambrose believed that taking up arms in defense was morally justifiable, as was the use of arms to punish those who committed atrocities against others. Further, the motivations and purposes of the war must align with God’s divine will, not the selfish ambitions of men. But he believed the Church and its clergy must not engage in violence themselves. Ambrose is significant here because for a time Augustine was a student at Ambrose’s school in Milan, and Ambrose’s sermons were instrumental in Augustine’s conversion to Christianity.

Augustine also drew upon earlier Roman conventions about war, for example, that it must be declared by proper authorities and not by angry mobs. The Empire also had a policy — not always followed — of allowing conquered people to keep their own customs, religions, and local laws, and extending to them at least some rights of Roman citizenship. This enabled conquered people to be integrated into the Empire. This system had clear advantages over treating people brutally if one wanted to enlarge an empire without fighting endless insurrections.

Augustine’s Just War: Jus ad bellum

Just war theory has two parts, labeled in Latin jus ad bellum and jus in bello. Jus ad bellum looks at when war is morally justifiable. Jus in bello looks at the conduct of war, and which actions are allowable and which are not.

Jus ad bellum, Augustine wrote, has two primary goals: to punish sinners and to restore peace. Punishing sinners must be done without vindictiveness, however. Jesus said, “But I tell you, love your enemies and pray for those who persecute you” (Matthew 5:44) and Augustine took this very seriously. Augustine wrote that the victor must “act with a sort of kindly harshness, when we are trying to make unwilling souls yield.” This punishment is of a corrective nature, not a vindictive one. And, of course, a just war requires that diplomacy be tried and exhausted before taking up arms.

Which wars are not just? These would be wars marked by selfish intent. Wars to seize territory, wealth, or power are not just. Wars fought for personal glory or out of vindictiveness are not just. Righteous leaders considering war would need to engage in serious introspection to be sure motivations are pure, I would think.

A just war must be declared by the proper authorities. In Augustine’s time, this would be an emperor or king. Augustine’s argument here is that it is the legal authority that determines if the war is just and necessary, and not individual soldiers. God’s judgment for starting a war would then fall only on the authority. Augustine did not explicitly address what an individual citizen should do if called to fight in a war that doesn’t seem just at all, but what he did say takes us into the second part of just war theory, jus ad bellum.

Just Conduct in War: Jus ad bello

Augustine did not focus on jus ad bello as directly as jus ad bellum, but I’m taking some liberties here and moving some of his bellum ideas into the bello category. And the first question we might ask is, how does the Christian soldier reconcile killing in war? In his great work City of God, Augustine wrote that “he who is commanded to perform this ministry does not himself slay. Rather, he is like a sword which is the instrument of its user.” In other words, in a just war the soldier is an instrument of God, not a murderer, and is not committing a sin.

Wars fought to engage in revengeful cruelty, or a love of violence are unjust, and so we might extrapolate from that soldiers should avoid these evils also. And if peace is the ultimate aim of a just war, soldiers should keep that aim in their hearts. Augustine does not directly address treatment of noncombatants and prisoners, which are important parts of modern just war theory. But remember, he was addressing pacifists.

Augustine’s Just War Today

As a species, we’ve walked a long road since the 4th century. I think most of us today would balk at engaging in war to punish evildoers, unless the evildoers are a clear and present threat to ourselves. And even then, we may want to think twice. It’s also the case that since the Nuremberg Trials of 1945 and 1946 we do expect soldiers to take moral responsibility for their conduct and refuse to follow orders to commit atrocities. War may be a continuation of politics by other means, but it’s a crude instrument that can spin out of control and lead to nothing but chaos and disorder.

But there is still wisdom for us in Augustine’s just war theory. In 418 he wrote to a nobleman named Boniface,

Think, then, of this first of all, when you are arming for the battle, that even your bodily strength is a gift of God; for, considering this, you will not employ the gift of God against God. For, when faith is pledged, it is to be kept even with the enemy against whom the war is waged, how much more with the friend for whom the battle is fought! Peace should be the object of your desire; war should be waged only as a necessity, and waged only that God may by it deliver men from the necessity and preserve them in peace. For peace is not sought in order to the kindling of war, but war is waged in order that peace may be obtained. Therefore, even in waging war, cherish the spirit of a peacemaker, that, by conquering those whom you attack, you may lead them back to the advantages of peace; for our Lord says: “Blessed are the peacemakers; for they shall be called the children of God.”

See also “Practical Just War: St. Augustine & His Framing of Just War Theory” by Benjamin Elkins, University of Pennsylvania

Augustine and just war theory

Saint Augustine of Hippo, painting attributed to Gerard Seghers (1591–1651). Source: Wikimedia Commons, public domain.

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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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