al-'Abbas was the head of one branch of the Banu Hashim, who traced their lineage to Hashim, an uncle of Muhammad. The Banu Hashim had great support from Shi'ites who thought that the family, which had produced Muhammad and 'Ali, would produce another great leader or mahdi who would liberate Islam. The half-hearted policies of the late Umayyads to tolerate non-Arab Moslems and Shi'ites had failed to quell unrest among these minorities.
This unrest led to revolt during the reign of Hisham in Kufa, a prominent city in southern Iraq. Shi'ite Moslems revolted in 736 and held the city until 740, led by Zayd ibn Ali, a grandson of Husayn and another member of the Banu Hashim. Zayd's rebellion failed, and was put down by Umayyad armies in 740. The revolt in Kufa indicated both the strength of the Umayyads and the growing unrest in the Moslem world.
al-'Abbas chose to focus on Khurasan, an important military city in eastern Iran. In 743, the death of the Umayyad Caliph provoked the third civil war. al-'Abbas, supported by Shi'ites, Kharijis, and the residents of Khurasan, led his forces to victory over the Umayyads and ultimately deposed the last Umayyad caliph, Marwan II in 750.
The civil war was marked by millenial prophecies encouraged by the beliefs of some Shi'ites that al-'Abbas was the mahdi. Prominent Islamic scholars wrote works such as the Jafr telling faithful Moslems that the brutal civil war was the great conflict between good and evil. The choice of the Umayyads to enter battle with white flags and the Abbasids to enter with black encouraged such theories. The color white, however, was regarded in much of Persia as a sign of mourning.
Concerned that there would never be a return of Umayyad power, al-'Abbas sought out all the remaining members of the Umayyad family to have them executed. Those that could escaped to al-Andalus, or Spain, led by Abd-ar-Rahman where the Umayyad caliphate would endure for three centuries. For his ruthless efforts to eliminate the Umayyad family, al-'Abbas gained the epithet al-Saffah, or blood-letter.
After the victory over the Umayyads, al-'Abbas's short reign was marked with efforts to consolidate and rebuild the Caliphate. His supporters were represented in the new government, but apart from his policy toward the Umayyad family, al-'Abbas is widely viewed by historians as having been a mild victor. Jews, Nestorian Christians, and Persians were well-represented in al-'Abbas's government and in succeeding Abbasid adminstrations.
Equally revolutionary was al-'Abbas's reform of the army, which came to include non-Moslems and non-Arabs in sharp contrast to the Umayyads who refused any soldiers that were either. al-'Abbas selected the gifted Abu Muslim as his military commander, an officer that would serve until 755 in the Abbasid army.
al-'Abbas reneged on his promises to the Shi'a community in claiming the Caliphate for himself. The Shi'ites had hoped that the imam would be named head of the Caliphate, inaugurating the era of peace and prosperity the millenialists ahd believed would come. The betrayal alienated al-'Abbas's Shi'ite supporters, although the continued amity of other groups made Abbasid rule markedly more solvent than Umayyad.
al-'Abbas died in 755 C.E., only five years after deposing the Umayyads. He was succeeded by his brother, al-Mansur.