The main cause of the First Crusade in 1096 was a combination of religious fervor and the desire for territorial expansion. Pope Urban II's call to arms aimed to aid the Byzantine Empire against Seljuk Turk invasions, as well as to liberate Jerusalem and other holy sites for Christian pilgrimage. The promise of spiritual rewards, including indulgences, motivated many knights and commoners to join the campaign. Economic factors, such as the prospect of land and wealth in the East, also played a significant role in attracting participants, ultimately leading to a fervent mobilization of support across Western Europe.