Crusading against Christians: The History and Legacy of the Catholic Church's Crusades against Other Christians during the Middle Ages

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 Eur...