After the fall of the Western Roman Empire in the 5th century, when Western Europe was governed by a Germanic warrior-caste, the theory of a just and virtuous war took root. The Roman Church enhanced its authority by sanctifying oaths taken for just military purposes, and Bishop Anselm of Lucca (d. 1086) was the first to suggest that military action for the cause of religion could remit sin. At the Council of Clermont in July 1095, Pope Urban II canonized religious war by urging Western Europe's nobility to take up arms in defense of the Byzantine Empire against the Muslims, thus launching the Crusades. Religious military orders such as the Knights of Saint John, the Templars, and the Hospitallers arose, ostensibly founded to protect the weak and the sick but also to extend the boundaries of Christianity and the power of the Church. In Europe, the knight, originally a mounted warrior, became a consecrated soldier of Christ, dedicated to the defense of the Church by solemn vows made before an altar.

Naturally, the question remained concerning the use of arms against other Christians. Eastern Christians did not acknowledge the Pope's supremacy, and many held that it was lawful for him to declare a crusade to bring schismatics back to the obedience of Rome. German knights fighting the Orthodox Russians at the Battle on the Ice in 1242 believed this, as did the Hungarian prosecutors of the 1235 invasion of Bosnia, which was thinly disguised as a crusade. The Church even extended the object of crusade to believers in communion with Rome, who refused to obey lawful authority. After peasants revolted against the Prince-Archbishop of Bremen in 1204 over tithes and land rights, Pope Gregory IX was persuaded to declare them heretics and proclaim a crusade against them.