The Crusades are a series of religious expeditionary wars blessed by Pope Urban II and the Catholic Church, with the stated goal of restoring Christian access to the holy places in and near Jerusalem, a sacred city and symbol of all three major Abrahamic faiths (Judaism, Christianity and Islam) The background to the Crusades is set when the Seljuk Turks decisively defeat the Byzantine army in 1071 and cut off Christian access to Jerusalem.

The Byzantine emperor, Alexis I, fears that all Asia Minor would be overrun.

He calls on western Christian leaders and the papacy to come to the aid of Constantinople by undertaking a pilgrimage or a crusade that would free Jerusalem from Muslim rule.

Another cause is the destruction of many Christian sacred sites and the persecution of Christians under the Fatimid caliph Al-Hakim.The crusaders comprise military units of Roman Catholics from all over western Europe, and are not under unified command.

The main series of Crusades, primarily against Muslims in the Levant, occur between 1095 and 1291.

Historians have given many of the earlier crusades numbers.

After some early successes, the later crusades fail and the crusaders are defeated and forced to return home.

Several hundred thousand soldiers become Crusaders by taking vows; the Pope grants them plenary indulgence.

Their emblem is the cross — the term "crusade" is derived from the French term for taking up the cross.

Many are from France and call themselves "Franks," which becomes the common term used by Muslims.

The term "crusade" is also used to describe religiously motivated campaigns conducted between 1100 and 1600 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 lead also to alliances between religious factions against their opponents, such as the Christian alliance with the Islamic Sultanate of Rûm during the Fifth Crusade.The Crusades have major political, economic, and social impact on western Europe and result in a substantial weakening of the Christian Byzantine Empire, which falls several centuries later to the Muslim Turks.

The Reconquista, a long period of wars in Spain and Portugal (Iberia), where Christian forces reconquer the peninsula from Muslims, is closely tied to the Crusades.