In the Middle Ages, the territories surrounding the Mediterranean Sea were divided into either Christian or Muslim regions. Relations between the two faiths were not friendly, and the status of Jerusalem was a special area of concern for Christians. Jerusalem was venerated as the holiest place in Christendom.
Christianity itself was divided, too. The Western Church was centered in Rome, and the Eastern Church was based in Constantinople. In 1054, the Great Schism formally divided Christendom. Although the Christians sometimes fought one another, they eventually united against their Muslim foes. In 1071, the Byzantines were crushed by a Muslim force at the Battle of Manzikert. The Byzantines were not able to recover from the disaster, so their new leader, Emperor Alexius, appealed to the Pope for assistance in 1095.
In Rome, Pope Urban welcomed the opportunity to buttress his power and prestige both in the West and vis-a-vis the Eastern Church. At Clermont, in 1095, the Pope proclaimed: "God wills it." He meant that Crusaders were answering God's call and would have their sins forgiven if they went to Jerusalem. The response was overwhelming, and the vast majority of participants were motivated by religious zeal. In 1099, the First Crusade was completed when Jerusalem fell to a Christian army.
The Crusaders' success was as ephemeral as their spirituality was dubious. When Jerusalem fell, Muslims, Jews, women, and children were slaughtered. Even the Byzantines rightfully distrusted their brutal allies. The Crusaders' leaders often fought among themselves and were preoccupied with temporal joys rather than religious duties. Jerusalem was recaptured by Muslims in 1187, and subsequent Crusades to retake it failed.
The enmity between Christians and Muslims would last for centuries and—in a sense—still endures today.