Saint Augustine, born in 354, is widely cited as the founder of Christian Just War Theory. This area of thought takes a look at situational ethics and tries to add temperance to war. Augustine looked at a violent world and refused the fantasy that moral people can always stay clean. Augustine’s role in this story is that he established the tradition of wrestling with the brutal question of war. He gave us the sense that even a necessary war is still a tragedy, not a triumph.

Centuries later, around 1265, Thomas Aquinas clarified his message:

“In order for a war to be just, three things are necessary.”

A Dominican friar in the 13th century, Aquinas was born about 1225, gave us his famous three necessary conditions for war: proper authority, just cause, and right intent. In simple terms, war cannot just be somebody’s anger with weapons or private revenge. It must be in defense and answer a real wrong. It must be declared by the proper authority, and aim at restoring justice or peace rather than feeding corruption, greed, or bloodlust.

From there, Just War thinking spread far beyond medieval Christianity. Its descendants helped shape the broader moral grammar of war across the world, especially through international humanitarian law. The basic idea is simple but powerful: even war has limits. You do not target civilians. You do not torture prisoners. You do not take hostages. You do not humiliate detainees. You do not use force without restraint. So, in modern times, beyond Christian thought, nations strive to put a moral leash on organized violence.