The question of when, if ever, it is morally permissible to go to war has been debated by Christians for nearly two millennia. 

The roots of Just War theory trace back to St. Augustine, who occasionally discussed the justice or injustice of war in writings such as Against Faustus. Augustine did not create a systematic theory, but his scattered reflections laid the groundwork for later thinkers. 

By the 12th century, the canonist Gratian, in his Decretum (c. 1140), cited Augustine dozens of times in discussing war, showing how the Church had begun to treat military action as a moral question, not just a political or practical one.

The first clear articulation of a systematic Just War theory came from St. Thomas Aquinas. In his Summa Theologiae (II-II, 40, 1), Aquinas wrote: “In order for a war to be just, three things are necessary: proper authority, just........

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