Crusades: The siege of Damascus ended in a decisive victory for the Muslims, leading to the disintegration of the Second Crusade.

The Crusades were a series of military campaigns launched by the papacy between 1095 and 1291 against Muslim rulers for the recovery and defence of the Holy Land, as part of a wider crusading movement. The First Crusade was proclaimed by Pope Urban II at the Council of Clermont in November 1095—a call to arms for Christians to reconquer Jerusalem from the Muslims, with promises of spiritual reward. By this time, the papacy's position as head of the Catholic Church had strengthened, and earlier conflicts with secular rulers and wars on the frontiers of Western Christendom had prepared it for the direction of armed force in religious causes. The successes of the First Crusade led to the establishment of four Crusader states in the Levant, where their defence required further expeditions from Catholic Europe. The organisation of such large-scale campaigns demanded complex religious, social, and economic institutions, including crusade indulgences, military orders, and the taxation of clerical income. Over time, the crusading movement expanded to include campaigns against pagans, Christian dissidents, and other enemies of the papacy, promoted with similar spiritual rewards and continuing into the 18th century.