How did Christian ideas in the Middle Ages influence early limits on warfare?

During the Middle Ages, Christian ideas significantly shaped early limits on warfare by fostering the development of just war theory and movements aimed at protecting noncombatants.1

Theological contributions, particularly from figures like St. Augustine and St. Thomas Aquinas, established frameworks for when armed force could be considered legitimate and how it should be conducted. These ideas were integrated into canon law, which helped codify moral restraints on violence.1

Additionally, the Peace of God movement, led by the church, sought to curb endemic violence by protecting vulnerable groups—such as women, clergy, pilgrims, and merchants—and ecclesiastical property. This movement utilized church councils and the invocation of saints to enforce peace, eventually influencing the Truce of God, which restricted the days on which warfare was permitted. Furthermore, the chivalric code reinforced these Christian ethics by promoting ideals of honor, courtesy, and the protection of the defenseless, which helped establish early norms for noncombatant immunity and restraint in the means of war.213