The Crusades were a series of religious wars initiated, supported, and sometimes directed by the Christian Latin Church in the medieval period. The best known of these military expeditions are those to the Holy Land in the period between 1095 and 1291 that had the objective of reconquering Jerusalem and its surrounding area from Muslim rule after the region had been conquered by the Rashidun Caliphate centuries earlier. Beginning with the First Crusade, which resulted in the conquest of Jerusalem in 1099, dozens of military campaigns were organised, providing a focal point of European history for centuries. Crusading declined rapidly after the 15th century.

A religious war or a war of religion, sometimes also known as a holy war, is a war which is primarily caused or justified by differences in religion and beliefs. In the modern period, there are frequent debates over the extent to which religious, economic, ethnic or other aspects of a conflict are predominant in a given war. The degree to which a war may be considered religious depends on many underlying questions, such as the definition of religion, the definition of 'religious war', and the applicability of religion to war as opposed to other possible factors. Answers to these questions heavily influence conclusions on how prevalent religious wars have been as opposed to other types of wars.