In 1095, Pope Urban II called for the First Crusade in a sermon at the Council of Clermont. He encouraged military support for the Byzantine Empire and its Emperor, Alexios I, who needed reinforcements for his conflict with westward migrating Turks colonizing Anatolia. One of Urban's aims was to guarantee pilgrims access to the Eastern Mediterranean holy sites that were under Muslim control but scholars disagree as to whether this was the primary motive for Urban or those who heeded his call. Urban's strategy may have been to unite the Eastern and Western branches of Christendom, which had been divided since the East–West Schism of 1054 and to establish himself as head of the unified Church. The initial success of the Crusade established the first four Crusader states in the Eastern Mediterranean: the County of Edessa, the Principality of Antioch, the Kingdom of Jerusalem and the County of Tripoli. The enthusiastic response to Urban's preaching from all classes in Western Europe established a precedent for other Crusades. Volunteers became Crusaders by taking a public vow and receiving plenary indulgences from the Church. Some were hoping for a mass ascension into heaven at Jerusalem or God's forgiveness for all their sins. Others participated to satisfy feudal obligations, obtain glory and honour or to seek economic and political gain.
The two-century attempt to recover the Holy Land ended in failure. Following the First Crusade there were six major Crusades and numerous less significant ones. After the last Catholic outposts fell in 1291, there were no more Crusades; but the gains were longer lasting in Northern and Western Europe. The Wendish Crusadeand those of the Archbishop of Bremen brought all the North-East Baltic and the tribes of Mecklenburg and Lusatia under Catholic control in the late 12th century. In the early 13th century the Teutonic Order created a Crusader state in Prussia and the French monarchy used the Albigensian Crusade to extend the kingdom to the Mediterranean Sea. The rise of the Ottoman Empire in the late 14th century prompted a Catholic response which led to further defeats at Nicopolis in 1396 and Varna in 1444. Catholic Europe was in chaos and the final pivot of Christian–Islamic relations was marked by two seismic events: the fall of Constantinople to the Ottomans in 1453 and a final conclusive victory for the Spanish over the Moors with the conquest of Granada in 1492. The idea of Crusading continued, not least in the form of the Knights Hospitaller, until the end of the 18th-century but the focus of Western European interest moved to the New World.
Modern historians hold widely varying opinions of the Crusaders. To some, their conduct was incongruous with the stated aims and implied moral authority of the papacy, as evidenced by the fact that on occasion the Pope excommunicated Crusaders. Crusaders often pillaged as they travelled, and their leaders generally retained control of captured territory instead of returning it to the Byzantines. During the People's Crusade, thousands of Jews were murdered in what is now called the Rhineland massacres. Constantinople was sacked during the Fourth Crusade. However, the Crusades had a profound impact on Western civilisation: Italian city-states gained considerable concessions in return for assisting the Crusaders and established colonies which allowed trade with the eastern markets even in the Ottoman period, allowing Genoa and Venice to flourish; they consolidated the collective identity of the Latin Church under papal leadership; and they constituted a wellspring for accounts of heroism, chivalry, and piety that galvanised medieval romance, philosophy, and literature. The Crusades also reinforced a connection between Western Christendom, feudalism, and militarism.
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