Abbasids. Name of a dynasty of Muslim caliphs. The Umayyads were overthrown by a combination of Shiite, Arab and non-Arab Muslims dissatisfied with the Umayyad regime. The rebels were led by the Abbasid family, descendants of the Prophet's uncle Abbas. From about 718 the Abbasids had plotted to take the caliphate, sending agents into various parts of the Muslim empire to spread propaganda against the Umayyads. By 747 they had secured enough support to organize a rebellion in northern Iran that led to the defeat of the Umayyad caliphate three years later. The Abbasids executed most of the Umayyad family, moved the capital of the empire to Baghdad, and assimilated much of the pomp and ceremony of the former Persian monarchy into their own courts.

Beginning in 750 with Abu al-Abbas, the Abbasid caliphate lasted five centuries; it is the most durable and most famous Islamic dynasty. The Abbasids became patrons of learning and encouraged religious observance. They were the first Muslim rulers to become leaders of an Islamic civilization and protectors of the religion rather than merely an Arab aristocracy imposing an Arab civilization on conquered lands. Under their caliphate Baghdad replaced Medina as the center of theological activity, industry and commerce developed greatly, and the Islamic empire reached a peak of material and intellectual achievement.

The 8th- and 9th-century caliphs Harun ar-Rashid and his son Abdullah al-Mamun are especially renowned for their encouragement of intellectual pursuits and for the splendor of their courts. During their reigns scholars were invited to the court to debate various topics, and translations were made from Greek, Persian, and Syriac works. Embassies also were exchanged with Charlemagne, emperor of the West.

In the late 9th century, the Abbasid caliphs increasingly began to delegate administrative responsibility to ministers of state and other government officials and to lose control over their Baghdad guards. As they gradually gave up personal political power, the caliphs placed more and more emphasis on their role as protectors of the faith. One result of this change in emphasis was the increased persecution of heretics and non-Muslims. About the same time, several successful revolts in the eastern provinces led to the establishment of independent principalities, and independent caliphates were subsequently established in North Africa and in Spain. Eventually, the power of the Abbasids barely extended outside Baghdad, and by the middle of the 10th century, the Abbasid caliphs had virtually no power, serving merely as figureheads at the mercy of the military commanders. The final defeat of the Abbasid dynasty came from outside the Muslim world, when al-Mustasim was put to death by the invading Mongols at the order of Hulagu, the grandson of Genghis Khan.