Battle of Manzikert
The battle of Manzikert (1071) in which the Byzantine army was defeated by the Seljuks Turks, is one of the decisive battles of history.
In the spring of 1071 the Byzantine emperor Romanus tried to extend his influence into parts of Turkish - held Armenia. His army was composed of mercenaries that included some Turkmen. The night before the battle his Turkmen went over to the enemy, and in the following battle the Byzantine army was utterly destroyed.
After Manzikert, there was nothing to oppose the Turkish flood, and only the Bosphorus saved the Empire. Anatolia was occupied by the Turks and the Byzantines were confined to the European part of the empire and a small strip of land along the Bosphorus.