The Crusades were a series of military conflicts of a religious character waged by much of Christian Europe against external and internal threats. Crusades were fought against Muslims, pagan Slavs, Russian and Greek Orthodox Christians, Mongols, Cathars, Hussites, Jews, and political enemies of the popes. Crusaders took vows and were granted an indulgence for past sins.
The Crusades originally had the goal of recapturing Jerusalem and the Holy Land from Muslim rule and were originally launched in response to a call from the Eastern Orthodox Byzantine Empire for help against the expansion of the Muslim Seljuk Turks into Anatolia. The term is also used to describe contemporaneous and subsequent campaigns conducted in territories outside the Levant usually against pagans, heretics, and peoples under the ban of excommunication for a mixture of religious, economic, and political reasons. Rivalries among both Christian and Muslim powers led also to alliances between religious factions against their opponents, such as the Christian alliance with the Sultanate of Rum during the Fifth Crusade.
The Crusades had far-reaching political, economic, and social impacts, some of which have lasted into contemporary times. Because of internal conflicts among Christian kingdoms and political powers, some of the crusade expeditions were diverted from their original aim, such as the Fourth Crusade, which resulted in the sack of Christian Constantinople and the partition of the Byzantine Empire between Venice and the Crusaders.