Why did Pope Urban II call the First Crusade?

Pope Urban II called for the First Crusade at the Council of Clermont in 1095, driven by a complex blend of geopolitical, religious, and ecclesiastical motivations.

The primary catalyst was a formal request for military aid from the Byzantine emperor Alexius I Comnenus. The Byzantine Empire had suffered significant territorial losses to the Seljuq Turks, most notably at the Battle of Manzikert in 1071, which left Asia Minor vulnerable and brought Muslim forces perilously close to Constantinople. Urban II viewed providing this military assistance as a potential pathway toward restoring ecclesiastical unity between the Eastern and Western branches of Christianity.1234

Beyond strategic concerns, the Pope was influenced by the popular religious climate of the 11th century:

• Protection of Pilgrims: Reports of the molestation of pilgrims and the desecration of holy sites in Jerusalem deeply concerned Western Christians.153
• Holy War and Penance: The Church increasingly framed the defense of Christendom as a holy work. By granting a plenary indulgence, Urban offered spiritual remission to those who fought to aid Eastern Christians, effectively channeling the warlike energies of the nobility toward a religious cause.253
• Christian Solidarity: There was a strong moral imperative to provide aid to fellow Christians in the East who were facing existential threats from the expansion of the Muslim world.13