Alarmed the Byzantine emperor appealed to Rome for help which presented the pope with a golden opportunity to reassert his supremacy over the eastern Church which had broken away from Rome in AD 1054.

At the same time pilgrimages to Jerusalem had encountered increased hostility from the Seljuks which led Pope Urban II to preach a Holy War against the heathens.

The first Crusade (1096-99) led to the successful storming of Jerusalem and the establishment of the kingdom of Jerusalem and other crusader states.

The second Crusade (1147-9) was led by the emperor Conrad III and by Louis VII of France, but foundered on quarrels between its leaders and the barons of Jerusalem, who were in alliance with Muslim Damascus, which the Crsuaders wished to attack.

The third Crusade (1189-92) was led by emperor Frederick I Barbarossa (who died before reaching Jerusalem); King Richard I of England and King Philip II Augustus of France. It failed to regain Jerusalem, which had been captured by the Muslim leader Saladin in 1187, but gained the concession that pilgrimages to Jersusalem were allowed.

Despite some successes the crusades failed to check the Turkish advance.

The climax came in 1204 when the Fourth Crusade became an attack on the Greek (Byzantine) empire instead of the Muslims. The Crusaders captured Constantinople and set up a short-lived Latin Empire (1204-61). The main beneficiaries were the Venetians who financed the enterprise and ended up by seizing all the islands that lay on the trade routes they now monopolized.

The Fifth Crusade (1228-29) was led by the emperor Frederick II. A truce between Frederick and Muslims secured Jerusalem for the Christians, but Pope Gregory IX disapproved of such trucemaking with the heathens, and severely disciplined Frederick. Soon the Muslim captured the city again and two more Crusades (1248-54, and 1270) did nothing toward freeing the Holy City.