Battle of the Zab

The Battle of the Zab took place on the banks of the Great Zab river in what is now Iraq on January 25, 750. It spelled the end of the Umayyad Caliphate and the rise of the Abbasids, a dynasty that would last (under various influences and with varying power) until the 13th century.

http://en.wikipedia.org/wiki/Battle_of_Zab