Military expeditions, beginning in the late 11th century, that were organized by Western Christians in response to centuries of Muslim wars of expansion.
Their objectives were to check the spread of Islam, to retake control of the Holy Land, to conquer pagan areas, and to recapture formerly Christian territories. The Crusades were seen by many of their participants as a means of redemption and expiation for sins. Between 1095, when the First Crusade was launched by Pope Urban II at the Council of Clermont, and 1291, when the Latin Christians were finally expelled from their kingdom in Syria, there were numerous expeditions to the Holy Land, to Spain, and even to the Baltic; the Crusades continued for several centuries after 1291, usually as military campaigns intended to halt or slow the advance of Muslim power or to conquer pagan areas.
The Crusaders initially enjoyed success, founding a Christian state in Palestine and Syria, but the continued growth of Islamic states ultimately reversed those gains. By the 14th century the Ottoman Turks had established themselves in the Balkans and would penetrate deeper into Europe despite repeated efforts to repulse them. Crusades were also called against heretics (the Albigensian Crusade, 120929) and various rivals of the popes, and the Fourth Crusade (120204) was diverted against the Byzantine Empire. Crusading declined rapidly during the 16th century with the advent of the Protestant Reformation and the decline of papal authority.
The Crusades constitute a controversial chapter in the history of Christianity, and their excesses have been the subject of centuries of historiography. Historians have also concentrated on the role the Crusades played in the expansion of medieval Europe and its institutions, and the notion of "crusading" has been transformed from a religio-military campaign into a modern metaphor for zealous and demanding struggles to advance the good ("crusades for") and to oppose perceived evil ("crusades against").