Byzantium had lost a lot of territory to the invading Seljuk Turks. In November 1095, at the Council of Clermont in southern France, the Pope called on Western Christians to take up arms to aid the Byzantines and recapture the Holy Land from Muslim control. This marked the beginning of the Crusades.
The city of Jerusalem is significant in a number of religious traditions, including the Abrahamic religions Judaism, Christianity, and Islam, which consider it a holy city