What Is Just War Theory?

The Pope, Trump, J.D. Vance, and Mike Johnson were all feuding over whether the war with Iran is morally legitimate. They’re all appealing to the same tradition: just war theory. But most people likely have no idea what it actually says.

The Church Was Not Always Pro-War

Here’s what’s interesting: just war theory hasn’t always been part of the Christian tradition. Early Christians and Christian leaders were largely what we today call pacifists. The church father Clement of Alexandria wrote this in the second century:

“If you enroll as one of God’s people, heaven is your country and God your lawgiver. And what are his laws? You shall not kill; you shall love your neighbor as yourself. To him that strikes you on the cheek, turn to him the other also… The church is an army that sheds no blood.”

The idea of Christians waging just wars was unthinkable when Christians were a persecuted minority in the Roman Empire.

Augustine’s Compromise

Everything changed when Christianity merged with the Roman Empire. Suddenly, Christians were running the empire. Theologians needed to figure out how to justify war—which had been an affair of the empire, not the church—to their fellow disciples. Augustine was among the most important thinkers in this camp.

Drawing on Roman thinkers like Cicero, and now burdened with political authority, Augustine proposed that an ethical war had to meet three criteria: a just cause—usually self-defense or the protection of the weak; a declaration by a legitimate authority; and a goal of peace and social stability, not conquest.

These are simple but rigorous principles that are quite difficult to satisfy. And here’s what gets lost in most political debates: Augustine still believed war was inherently sinful, even when necessary. The Christian pacifist tradition never stopped influencing his thinking. He would not celebrate or glorify war the way many of us do today.

Aquinas and the Modern Framework

The medieval theologian Thomas Aquinas, though, disagreed. Aquinas still believed war is evil—but only in the sense that it is tragic and destructive, like a natural disaster. Killing is not inherently sinful if done for the sake of defending the common good. As the just war tradition matured, Christians became more and more comfortable with war—which is a far cry from the position of early Christians.

Aquinas added an important concept to just war theory: the principle of double effect. It stipulates that unintended evil consequences of an action may be permitted insofar as it meets four conditions: 1) the evil outcome is not the intended outcome; 2) the action taken to achieve the good end is proportionate—meaning you are not doing more harm than good; 3) the good outcome cannot depend on the bad outcome; and 4) the action itself cannot be intrinsically evil.

A good example is the killing of innocent people in war—sometimes callously called collateral damage. If destroying an enemy target is needed to end a war that will kill billions, it may be permitted to destroy it, even if there are innocent people near the target. But the principle requires the commander to do everything possible to avoid civilian deaths. Military necessity alone does not justify the killing of noncombatants.

Later just war theory incorporated these principles to give us the following updated criteria. A just war must have: 1) a just cause—self-defense or defense of the vulnerable; 2) declaration by a legitimate authority; 3) the right intention, namely peace; 4) last resort—diplomacy and other nonmilitary solutions are preferred first; 5) a good probability of success; and 6) proportionality—the damage and costs must not outweigh the good that could come from it. Two important principles govern conduct during a war: discrimination and legitimate targets. These tell us that militaries cannot morally target civilians or civilian infrastructure necessary for life. As you may have already realized, these principles helped shape the Geneva Conventions.

Notice how nothing here justifies a preemptive war of aggression against another nation. Notice how none of this justifies a direct attack on civilians and civilian infrastructure. Nothing here justifies threats of annihilation. Critics say just war theory has historically served to justify war rather than prevent it. I tend to agree.

Just war tradition is a very rigorous set of principles. Do you think any modern war could ever meet these criteria? Could war ever be moral? Let me know what you think in the comments below.