The Battle of Manzikert occurred on August 26, 1071 between Byzantine forces and Turkish forces, resulting in the crushing defeat of the Byzantine Empire and the capture of Emperor Romanus IV Diogenes.
Historians are practically unanimous in dating the decline of Byzantine fortunes to this battle, as well as in considering it one of the root causes for the later Crusades.
The Seljuk Turks were Muslim, and the battle was on a Friday.
