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Military Defeat, Financial Crisis, and Revolts
An unclear line of succession plagued the Umayyad dynasty throughout its reign, and civil unrest and tribal warfare often surrounded the naming of new caliphs. A clear decline began with the disastrous defeat of the Syrian army by the Byzantine emperor Leo III in 717. Soon afterward, caliph ?Umar II (reigned 717–20) initiated fiscal reforms as a response to complaints from the increasingly discontented mawālī (non-Arab Muslims). This well-intended attempt to place all Muslims on the same footing led to financial crisis. Meanwhile, feuds between southern (Kalb) and northern (Qays) Arab tribes reduced military power and erupted into major revolts in 745. The mawālī became involved with the Hāshimiyyah, a religio-political faction that denied the legitimacy of Umayyad rule. In 749 the Hāshimiyyah, aided by the western provinces, proclaimed as caliph Abū al-?Abbās al-Saffā? of the ?Abbasid family.
Loss to the ?Abbasids
The ?Abbasids were descended from an uncle of Muhammad. Seeing the weaknesses of the Umayyads, they declared a revolt in 747. With the help of a coalition of Persians, Iraqis, and Shī?ites, they put an end to the Umayyad dynasty with a victory against them at the Battle of the Great Zab River in 750. The last Umayyad caliph, Marwān II, was killed while fleeing the forces of Abū al-?Abbās al-Saffā?.