Abbasid was the dynastic name generally given to the caliphs of Baghdad, the second of the two great Sunni dynasties of the Muslim empire, that overthrew the Umayyid caliphs. It seized power in 758, when it finally defeated the Umayyads in battle, and flourished for two centuries, but slowly went into eclipse with the rise to power of the Turkish army they had created, the Mamluks. Their claim to power was finally ended in 1258, when Hulagu Khan, the Mongol general, sacked Baghdad. While they continued to claim authority in religious matters from their base in Egypt, their dynasty was ended.
Revolt against the Umayyads
The Abbasid caliphs officially based their claim to the Caliphate on their descent from Abbas ibn Abd al-Muttalib (AD 566-652), one of the youngest uncles of the Prophet Muhammad, by virtue of which descent they regarded themselves as the rightful heirs of the Prophet as opposed to the Umayyads. The Umayyads were descended from Umayya, and were a clan separate from Muhammad's in the Quraish tribe.
The Abassids also distinguished themselves from the Umayyads by attacking their secularism, their moral character and their administration in general. The Abassids also appealed to non-Arab Muslims, known as mawali , who remained outside the kinship-based society of Arab culture and were at best second-class citizens within the Umayyad empire. Muhammad ibn 'Ali, a great-grandson of Abbas, began to campaign for the return of power to the family of the Prophet, the Hashimites, in Persia during the reign of Umar II, Muhammad ibn Ali.
During the reign of Marwan II this opposition culminated in the rebellion of Ibrahim the Imam, the fourth in descent from Abbas, who, supported by the province of Khorasan, achieved considerable successes, but was captured (AD 747) and died in prison (as some hold, assassinated). The quarrel was taken up by his brother Abdallah, known by the name of Abu al-'Abbas as-Saffah, who, after a decisive victory on the Greater Zab river (750), finally crushed the Umayyads and was proclaimed Caliph.
Consolidation and schisms
The Abassids had depended heavily on the support of Persians in their overthrow of the Umayyads. Abu al-'Abbas' successor, Mansur, moved their capital from Damascus to the new city of Baghdad and welcomed non-Arab Muslims to their court. While this helped integrate Arab and Persian cultures, it alienated many of their Arab supporters, particularly the Khorosanian Arabs who had supported them in their battles against the Umayyads.
These fissures in their support led to immediate problems. The Umayyads, while out of power, were not destroyed. Reestablishing themselves in Spain in 756, they not only declared a rival caliphate, but sponsored a Moorish culture that was dramatically different from the fusion of Arab and Persian culture under the Abassids.
The Abassids also found themselves at odds with the Shia, many of whom had supported their war against the Umayyads, since the Abassids claimed legitimacy by their familial connection to Muhammed. Once in power, the Abassids embraced Sunni Islam and disavowed any support for Shi'a beliefs. That led to numerous conflicts, culminating in an uprising in Mecca in 786, followed by widespread bloodshed and the flight of many Shi'a to the Maghreb, where the survivors established the Idrisid kingdom. Shortly thereafter Berber Kharjites set up an independent state in North Africa in 801.
At the same time the Abassids faced challenges closer to home. The Byzantine Empire was fighting Abassid rule in Syria and Anatolia. Former supporters of the Abassids had broken away to create a separate kingdom around Khorosan in northern Persia. Harun al-Rashid (786 - 809) added to these troubles by turning on the Barmakids, the Persian family that had supplied the caliphate with competent administrators, over a personal dispute.
Faced with these challenges from within, the Abassids decided to create an army loyal only to their caliphate, drawn mostly from Turkish slaves, known as Mamluks, with some Slavs and Berbers participating as well. This force, created in the reign of al-Ma'mun (813 - 833), and his brother and successor al-Mu'tasim (833 - 842), prevented the further distintegration of the empire.
It also, however, led to the ultimate eclipse of Abassid rule. The creation of this foreign army and al-Mu'tasim's transfer of the capital from Baghdad to Samarra created a division between the caliphate and the peoples they claimed to rule. In addition, the power of the Mamluks steadily grew until al-Radi (934 - 941) was constrained to hand over most of the royal functions to Mahommed bin Raik. In the following years the Buyids, who were Shi'ites, seized power over Baghdad, ruling central Iraq for more than a century before being overthrown by the Seljuq Turks. In the same period, the Hamdanids, another Shi'ite dynasty, came to power in northern Iraq, leading to a tremendous expansion of Shi'a influence. In the process the Abassid caliphs became no more than figureheads.
Learning under the Abassid dynasty
The reigns of Harun al-Rashid (786 - 809) and his successors fostered an age of great intellectual achievement. In large part this was the result of the schismatic forces that had undermined the Umayyad regime, which relied on the assertion of the superiority of Arab culture as part of its claim to legitimacy, and the Abassids' welcoming of support from non-Arab Muslims.
A number of medieval thinkers and scientists living under Islamic rule, many of them non-Muslims or heretical Muslims, played a role in transmitting Greek, Hindu, and other pre-Islamic knowledge to the Christian West. They contributed to making Aristotle known in Christian Europe. In addition the period saw the recovery of much of the Alexandrian mathematical, geometric and astronomical knowledge, such as that of Euclides and Claudius Ptolemy, and these recovered mathematical methods were later enhanced and developed by other Islamic scholars, notably by Al-Biruni, and Abu Nasr Mansur, who are thought to have first derived the Cosine rule and applied it to spherical geometry.
Three speculative thinkers, the Persians al-Kindi, al-Farabi, and Avicenna, combined Aristotelianism and Neoplatonism with other ideas introduced through Islam.
The end of the caliphate
Hulagu Khan sacked Baghdad on (February 10, 1258), causing great loss of life. Al-Musta'sim, the last reigning Abbasid caliphate in Baghdad was then executed on February 20, 1258. The Abbasids still maintained a feeble show of authority, confined to religious matters, in Egypt under the Mamelukes, but the dynasty finally disappeared with Motawakkil III , who was carried away as a prisoner to Constantinople by Selim I.
See also History of Islam
Initial text from 1911 encyclopedia, as modified -- Please update as needed
Abbasid Caliphs of Baghdad
Abbasid Caliphs in Cairo