The Crusades were a series of religious wars initiated, supported, and sometimes directed by the Latin Church in the medieval period. The best known of these Crusades are those to the Holy Land in the period between 1095 and 1291 that were intended to recover Jerusalem and its surrounding area from Islamic rule. Concurrent military activities in the Iberian Peninsula against the Moors (the Reconquista) and in northern Europe against pagan West Slavic, Baltic and Finnic peoples (the Northern Crusades) also became known as crusades. Through the 15th century, other church-sanctioned crusades were fought against heretical Christian sects, against the Byzantine and Ottoman empires, to combat paganism and heresy, and for political reasons. Unsanctioned by the church, Popular Crusades of ordinary citizens were also frequent. Beginning with the First Crusade which resulted in the recovery of Jerusalem in 1099, dozens of Crusades were fought, providing a focal point of European history for centuries.

In 1095, Pope Urban II proclaimed the First Crusade at the Council of Clermont. He encouraged military support for Byzantine emperor Alexios I against the Seljuk Turks and called for an armed pilgrimage to Jerusalem. Across all social strata in western Europe, there was an enthusiastic popular response. The first Crusaders had a variety of motivations, including religious salvation, satisfying feudal obligations, opportunities for renown, and economic or political advantage. Later crusades were generally conducted by more organized armies, sometimes led by a king. All were granted papal indulgences. Initial successes established four Crusader states: the County of Edessa; the Principality of Antioch; the Kingdom of Jerusalem; and the County of Tripoli. The Crusader presence remained in the region in some form until the fall of Acre in 1291. After this, there were no further crusades to recover the Holy Land.