The Byzantine Empire began the 11th century in a strong position - they had pushed their frontier eastwards against the fragmented Muslim emirates, and had completely destroyed the Bulgars in the Balkans. All that was to change in 1071, when they suffered a catastrophic defeat at the hands of the Seljuq Turks at the Battle of Manzikert. These nomadic conquerors had recently converted to Islam, and had swiftly established a Sultanate ruling from Afghanistan to Palestine. Following Manzikert they took nearly all of Anatolia from the Byzantines.
As the Byzantine Emperor Alexios Komnenos struggled to stem the Seljuq advances, he appealed to the West for mercenaries. This request was seized upon by Pope Urban II, who possibly saw it as an opportunity to further his own aims. At the Council of Clermont in 1095 he called for a Crusade to save the eastern churches and recover the Holy Land from the Muslims.
The timing was fortuitously right, as the mighty Seljuq Empire had begun to fragment, the Sultanate of Rûm in Anatolia (modern Asiatic Turkey) having seceded from the Great Seljuk Empire in 1077, and the local Syrian atabegs being in practice semi-independent and disunited. The First Crusade eventually captured Jerusalem in 1099, and established a number of Crusader states in Palestine and Syria. In doing so they created bitter resentment between Muslims, Western Christians and the Byzantines that would lead to two centuries of conflict.
Several major Crusades were to follow the First, as the Crusader states fought for their existence against a succession of resurgent Islamic states: the Fatimids, Zangids, Ayyubids, and finally the Mamluks, who extinguished the last Crusader stronghold of Acre in 1291.