The early Church's posture toward war shifted dramatically between the pre-Constantinian period — when most writers counseled nonparticipation in Roman military service — and the post-Constantinian era, when Augustine developed the first systematic Christian just war theory. Augustine drew on Cicero's criteria and baptized them within a theology of love: war can be just when it aims at restoring peace, is authorized by legitimate authority, and is waged with the intention of benefiting even the enemy. Aquinas systematized Augustine's criteria into the three requirements that became the medieval foundation: just cause, right intention, and legitimate authority.
"In this Moses not only did not sin, but it would have been sin not to do it. It was by the command of God, who, from His knowledge both of the actions and of the hearts of men, can decide on what every one should be made to suffer, and through whose agency" — Augustine grounds Old Testament warfare in divine command, establishing that God-authorized violence is not sin but obligation.
"In order for a war to be just, three things are necessary. First, the authority of the sovereign by whose command the war is to be waged. For it is not the business of a private individual to declare war" — Aquinas's three-part framework of sovereign authority, just cause, and right intention that became the medieval foundation.
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