The Abbasids came to power under the auspices of a Shi`ite movement which, they claimed, had transferred its loyalty to Muhammad b. `Ali, the great-grandson of Muhammad's uncle `Abbas. Intensive propaganda began around 718 in Iraq and Khurasan. Muhammad was succeeded to his claims by his son Ibrahim, who decided to concentrate his efforts on Khurasan; in 745 he sent his Persian mawla, Abu Muslim, as his personal representative to Khurasan. The decision paid off: Abu Muslim was able to create a solid base of support and in 747 began the rebellion that would quickly lead to the end of the Umayyad caliphate. The death of Ibrahim in 748 after his capture by the Umayyads could not halt the steady westward procession of Abbasid forces: in 749 they reached Iraq and declared Ibrahim's brother, Abu al-Abbas, to be the new caliph with the title al-Saffah. The last Umayyad caliph was killed in 750.
It was under al-Mansur (754-775) that the changes brought by the Abbasid revolution were made manifest. Despite the Arab origins of the dynasty and the use of Arabic as the official language, the Arabs quickly lost the political and social superiority they had retained under the Umayyads; political prestige was increasingly determined by one's standing with the ruler. The seat of power was transferred from Syria to Iraq with the building of the city of Baghdad. Administration was placed in the hands of the Persian Barmakid family. Al-Mansur renounced the Shi`ite origins of the movement, stressing instead the Abbasids' own relationship to Muhammad through his uncle Abbas; Abu Muslim was put to death. The Abbasids in fact quickly became the champions of Sunni orthodoxy, a policy which helped them to unify an increasingly cosmopolitan Muslim empire. Under his son al-Mahdi (775-785), Sasanid Persian models of court etiquette and government were introduced into the Abbasid style of rulership.
The reign of Harun al-Rashid (786-809) is widely regarded as the apex of Abbasid power and achievement, but it is during his reign that one begins to see signs of weakness. Revolts in Persia were increasing in severity, while in the west the Abbasids lost their hold on eastern Morocco and Tunisia. To the north, the state was on the defensive against the Byzantines in Syria and Anatolia and against the Khazars in Armenia. Weaknesses within the caliphate were also manifest: the Barmakids were disgraced and thrown out of office, thus removing a loyal family of competent administrators.
After Harun's death, civil war broke out between his two sons, al-Amin, based in Iraq, and al-Ma'mun, based in Persia, which was resolved with al-Amin's death in 813. The founding of the Tahirid dynasty in 820 in Khurasan by al-Ma'mun's Persian general Tahir inspired others with similar aspirations; although this dynasty and the ones that followed it recognized the suzerainty of the Abbasids, they effectively eroded Abbasid authority in Persia. In what was probably an attempt to placate the Shi`ites, al-Ma'mun officially endorsed the Mu`tazilite theological school and attempted to force its doctrine on all religious scholars. This policy, which was continued until the advent of al-Mutawakkil's caliphate in 847, further alienated the caliph from his subjects.
Even in Iraq Abbasid authority was fading. Extravagent expenditures, an inflated bureaucracy, and dwindling revenues produced financial chaos. The caliphs attempted to solve this problem by granting tax-farms to governors and military commanders, thus effectively decentralizing and reducing Abbasid authority. The caliph al-Mu`tasim (833-842) is credited with importing Turkish slaves to serve as soldiers; their power quickly grew and eventually reduced the caliphate to a puppet institution. In 836, al-Mu`tasim transferred the seat of the caliphate to Samarra, some 60 miles north of Baghdad; his action signifies the growing gulf between the caliph and his subjects. The caliphate remained in Samarra until 892, when al-Mu`tamid returned to Baghdad.
A serious attempt to reinstate Abbasid power was made by al-Mutawakkil (847-862), who attempted to break the power of the Turkish military. After his murder in 862, a period of anarchy followed until the accession of al-Mu`tamid in 870. Al-Mu`tamid himself was not an effective ruler, but his brother al-Muwaffaq became the true power behind the throne and did much to reestablish Abbasid power. In Southern Iraq, black African slaves, known as the Zanj, began a rebellion that by 877 reached perilously close to Baghdad. Al-Muwaffaq gradually pushed the Zanj south and finally crushed their rebellion in 883. Meanwhile, al-Muwaffaq also checked the power of the Persian Tahirid and Saffarid dynasties.
Al-Muwaffaq's successful reassertion of caliphal power continued under his brother's two successors. However, with the reign of al-Muqtadir (908-932), who acceeded when only a boy of thirteen, the caliphate again slipped into decline. Qarmatian revolts disrupted trade and communications in Syria, southern Arabia, and Bahrayn, while in the west the Fatimid anti-caliphate was established in Tunsia and quickly began to encroach upon Abbasid territory in Egypt. Meanwhile, the Bedouin Hamdanid dynasty was established in Syria and northern Iraq, and the Shi`ite Buyid dynasty came to power in western Persia. The completion of the decline of Abbasid authority is symbolized by the granting of the title "Commander of Commanders" to the governor of Iraq: although probably intended to assert the superiority of the military commander of Baghdad over other commanders, this title effectively recognized the existence of temporal authority which left the caliph only as a formal head of state and representative of the Muslim community. In 945, the Buyid Mu`izz al-Dawla entered Baghdad, and the title "Commander of Commanders" and control of the Abbasid seat of power passed into the hands of a Shi`ite dynasty.
From the time of the Buyid occupation of Baghdad until its destruction by the Mongols in 1258, the Abbasid caliphate remained a purely formal institution. After the dissolution of the Buyids in the mid-eleventh century, their place was filled by the Turkish Seljuqs, who took the title of Sultan. Their rule reunified the Muslim state from Syria to Tranoxiana and stamped out the last Shi`ite revolutionary movements in the area of their control; these actions helped to enhance the prestige of the caliphate against their Fatimid rivals in Egypt. With the breakup of the Seljuq sultanate in the twelfth century, a power vacuum was left in Iraq enabled the caliph al-Nasir (1180-1225) to make an attempt to restore Abbasid power. However, his successors were incompetent, and the last caliph in Iraq, al-Mu`tasim, was unable to offer any resistence to the Mongols when they arrived in Baghdad in 1258.
The Abbasid line was reestablished by the Mamluks in Egypt in 1261. Although this caliphate was purely nominal, it served to legitimize the rule of secular dynasties. The last Abbasid caliph was deposed by Selim I, the Ottoman conquerer of Syria and Egypt; thereafter the title of caliph was held by the Ottoman sultans.
The `Abbasid Caliphate 132/750-170/786, F. Omar, Baghdad 1969
The `Abbasid Revolution, M.A. Shaban, Cambridge 1970
Black Banners from the East, M. Sharon, Jerusalem 1983
The Cambridge History of Islam, ed. P.M. Holt, Ann K.S. Lambton, and Bernard Lewis
The Encyclopaedia of Islam, second edition
The Shaping of `Abbasid Rule, J. Lassner, Princeton 1980