On August 26, 1071, the Seljuq Turks defeated the Byzantine Empire in a battle near Manzikert (modern Malazgirt in Muş Province, Turkey). The decisive defeat of the Byzantine army and the capture of the Emperor Romanos IV Diogenes played an important role in undermining Byzantine authority.
This eventually led to the Crusades.
Image: Muhammad bin Dawud Chaghri (Alp Arslan) humiliating Emperor Romanos IV. From a 15th-century illustrated French translation of Boccaccio’s De Casibus Virorum Illustrium – Bibliothèque nationale de France.