During the reign of Umayyad caliph Marwan II, the opposition culminates in the rebellion of Ibrahim the Imam, the fourth in descent from Abbas ibn Abd al-Muttalib (566 – 662), one of the youngest uncles of Muhammad.

Supported by the province of Khorasan, Iran, he achieves considerable successes, but is captured in the year 747 and dies in prison; some hold that he was assassinated.

The quarrel is taken up by his brother Abdallah, known by the name of Abu al-'Abbas as-Saffah, who, with victory on the Greater Zab River (750), defeats the Umayyads and is proclaimed Caliph.

The Battle of the Zab takes place on the banks of the Great Zab river in what is now Iraq on January 25, 750.

It spells the end of the Umayyad Caliphate and the rise of the Abbasids, a dynasty that is to last (under various influences and with varying power) until the 13th century.When the Abbasids declare amnesty for members of the Umayyad family, eighty gather to receive pardons, and all are massacred.