The Umayyad Caliphate continued to rule the Muslim world as a unified empire until the middle of the eighth century. Owing to the growing discontent because of the financial and administrative oppression and mismanagement of the Umayyad governors, a series of rebellions under the leadership of an Iranian commander named Abu Muslim was initiated from Khorasan, in the eastern Iranian lands. Gathering under the black banner of the Abbasids, relatives of the prophet and claimants to spiritual, as well as earthly, authority, this movement, known as the Abbasid Revolution, eventually managed to dislodge the Umayyads from their position of power. Decline began with the disastrous defeat of the Syrian army by the Byzantine emperor Leo III (the Isaurian; 717). Then the fiscal reforms of the pious ʿUmar II (reigned 717–720), intended to mollify the increasingly discontented mawālī (non-Arab Muslims) by placing all Muslims on the same footing regardless of ethnicity, led to financial crisis, while the recrudescence of feuds between southern (Kalb) and northern (Qays) Arab tribes seriously reduced military power (Source: Iranologie.com/Britanica).
General Division of Umayyad Caliphate: Sufyanids and Marwanids
