The Fourth Fitna, Fourth Muslim Civil War, or Great Abbasid Civil War resulted from the conflict between the brothers al-Amin and al-Ma'mun over the succession to the throne of the Abbasid Caliphate. Their father, Caliph Harun al-Rashid, had named al-Amin as the first successor, but had also named al-Ma'mun as the second, with Khurasan granted to him as an appanage. Later a third son, al-Qasim, was designated as third successor. After Harun died in 809, al-Amin succeeded him in Baghdad. Encouraged by the Baghdad court, al-Amin began trying to subvert the autonomous status of Khurasan, and al-Qasim was quickly sidelined. In response, al-Ma'mun sought the support of the provincial élites of Khurasan and made moves to assert his own autonomy. As the rift between the two brothers and their respective camps widened, al-Amin declared his own son Musa as his heir and assembled a large army. In 811, al-Amin's troops marched against Khurasan, but al-Ma'mun's general Tahir ibn Husayn defeated them in the Battle of Ray, and then invaded Iraq and besieged Baghdad itself. The city fell after a year, al-Amin was executed, and al-Ma'mun became Caliph.
Al-Ma'mun chose to remain in Khurasan, however, rather than coming to the capital. This allowed the power vacuum which the civil war had fostered in the Caliphate's provinces to grow, and several local rulers sprang up in Jazira, Syria and Egypt. In addition, a series of Alid uprisings occurred, beginning with Abu'l-Saraya at Kufa and spreading to southern Iraq, the Hejaz, and Yemen. The pro-Khurasani policies followed by al-Ma'mun's powerful chief minister, al-Fadl ibn Sahl, and al-Ma'mun's eventual espousal of an Alid succession in the person of Ali al-Ridha, alienated the traditional Baghdad élites, who saw themselves increasingly marginalized. Consequently, al-Ma'mun's uncle Ibrahim was proclaimed rival caliph at Baghdad in 817, forcing al-Ma'mun to intervene in person. Fadl ibn Sahl was assassinated and al-Ma'mun left Khurasan for Baghdad, which he entered in 819. The next years saw the consolidation of al-Ma'mun's authority and the re-incorporation of the western provinces against local rebels, a process not completed until the pacification of Egypt in 827. Some local rebellions, notably that of the Khurramites, dragged on for far longer, into the 830s.
