The Fourth Fitna 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, had been 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.
"The victory of Maʿmun over Amin". Folio from a manuscript of Nigaristan, Iran, probably Shiraz, dated 1573–74.
The populace pays Allegiance to the Abbasid caliph, Al-Ma'mun in 813. (from the book Tarikh-i Alfi 1593 CE)
The Imam Reza shrine, erected over the grave of Ali ibn Musa al-Ridha
The Caliph al-Ma'mun (left) as depicted in the 13th-century Madrid Skylitzes manuscript, seen receiving the embassy of John the Grammarian in 829, sent by the Byzantine emperor Theophilos (depicted right)
Abu Musa Muhammad ibn Harun al-Rashid, better known by his laqab of al-Amin, was the sixth Arab Abbasid caliph from 809 to 813.
Gold dinar of al-Amin, minted in Baghdad in 811
Dinar of Harun minted in Baghdad 184 AH (800 CE) with the name of Commander of the Faithful Harun al-Rashid and his first Heir, prince al-Amin
Bukharan coinage, naming al-Amin as governor of Khurasan, c. 796–801
The victory of al-Maʿmun over al-Amin. Folio from a manuscript of Nigaristan, probably Shiraz, dated 1573–1574.