As-Saffah is the head of one branch…

As-Saffah is the head of one branch of the Banu Hāshim from Arabia, a subclan of the famous Quraysh tribe who trace their lineage to Hāshim, a great-grandfather of the Prophet Muhammad via 'Abbās, an uncle of the Prophet, hence the title "Abbasid" for his descendants' caliphate.

This indirect link to the Prophet's larger clan forms sufficient basis for As-Saffah's claim to the title caliph.

However, the tradition that 'Abbās himself never converted to Islam or only halfheartedly weakens that legitimacy in some eyes.

As narrated in many hadith, many believed that in the end times a great leader or mahdi would appear from the family of the Prophet Muhammad, to which Ali belonged, who would deliver Islam from corrupt leadership.

The halfhearted policies of the later Umayyads to tolerate non-Arab Muslims and Shi'as have failed to quell unrest among these minorities.

During the reign of late Umayyad Caliph Hisham ibn Abd al-Malik, this unrest has led to revolt in Kufa, now a prominent Muslim city in southern Iraq.

Shi'ites had revolted in 736 and hold the city until 740, led by Zayd ibn Ali, a grandson of the famous martyr Husayn and another member of the Banu Hashim.

Zayd's rebellion fails, and is put down by Umayyad armies in 740.

The revolt in Kufa indicates both the strength of the Umayyads and the growing unrest in the Muslim world.