The Crusades were a series of military campaigns of a religious character waged by much of Christian Europe against external and internal opponents. Crusades were fought mainly against Muslims, though campaigns were also directed against pagan Slavs, Jews, Russian and Greek Orthodox Christians, Mongols, Cathars, Hussites, Waldensians, Old Prussians and political enemies of the popes. Crusaders took vows and were granted an indulgence for past sins.Riley-Smith, Jonathan. The Oxford History of the Crusades New York: Oxford University Press, 1999. ISBN 0192853643.

The Crusades originally had the goal of recapturing Jerusalem and the Holy Land from Muslim rule and were 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 through to the 16th century 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. The Sixth Crusade was the first crusade to set sail without the official blessing of the Pope. The Seventh, Eighth and Ninth Crusades resulted in Mamluk and Hafsid victories, as the Ninth Crusade marked the end of the Crusades in the Middle East.

Middle Eastern situation

The Muslim presence in the Holy Land began with the initial Arab conquest of Palestine in the 7th century. The Muslim armies' successes put increasing pressure on the Eastern Orthodox Byzantine Empire.

Another factor that contributed to the change in Western attitudes towards the East came in the year 1009, when the Fatimid Caliph al-Hakim bi-Amr Allah ordered the destruction of the Church of the Holy Sepulchre. In 1039 his successor, after requiring large sums be paid for the right, permitted the Byzantine Empire to rebuild it. Pilgrimages were allowed to the Holy Lands before and after the Sepulchre was rebuilt, but for a time pilgrims were captured and some of the clergy were killed. The Muslim conquerors eventually realized that the wealth of Jerusalem came from the pilgrims; with this realization the persecution of pilgrims stopped. However, the damage was already done, and the violence of the Seljuk Turks became part of the concern that spread the passion for the Crusades.