the abbasid dynasty comes to power after the umayyad army is defeated at the battle of the zab
The Battle of the Zab (Arabic: معركة الزاب‎) 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.
Battle of the Zab