The Crusades were a series of several military campaigns—usually sanctioned by the Papacy—that took place during the 11th through 13th centuries. Originally, they were Roman Catholic endeavors to recapture Jerusalem and the Holy Land from the Muslims, but some were directed against other Europeans, such as the Fourth Crusade against Constantinople, the Albigensian Crusade against the Cathars of southern France and the Northern Crusades.
The First Crusade began after Byzantine emperor Alexius I called for help with defending his empire against the Seljuk Turks. In 1095 at the Council of Clermont Pope Urban II called upon all Christians to join a war against the Turks, a war which would count as full penance.
The Crusades had an enormous influence on the European Middle Ages. By the 14th century the old concept of Christendom was fragmented,  European castles had become massive stone structures and the need to raise, transport and supply large armies led to a flourishing of trade throughout Europe.
The crusades had profound but localized effects upon the Western Islamic world, where the equivalents of "Franks" and "Crusaders" remained expressions of disdain. Muslims traditionally celebrate Saladin, the Kurdish warrior, as a hero against the Crusaders. The Crusades were regarded as cruel and savage onslaughts by European Christians.
The Crusaders' atrocities against Jews in the German and Hungarian towns, later also in those of France and England, and in the massacres of non-combatants in Palestine and Syria have become a part of history of anti-Semitism. The social position of the Jews in western Europe was distinctly worsened, and legal restrictions increased during and after the Crusades.
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