Haroun al-Rashid had decided the succesion to his sons during a pilgrimage to Mecca. The eldest, al-Amin, would receive the Caliphate and the younger, al-Ma'mun, would become governor of Khurasan in eastern Iran. On al-Amin's death, according to Haroun's decision, al-Mamun would become Caliph.
Shortly after Haroun died in 809 and al-Amin was declared caliph, al-Amin announced that his son would inherit rather than al-Ma'mun, provoking the fourth civil war.
al-Ma'mun, whose mother was Persian, received major support from Iran, and as governor, the military center of Khurasan. Playing himself as a champion of Persian liberties, the Iranian plateau united behind him. His faithful general, Tahir (d. 822) led his armies into Iraq.
al-Amin appealed to his mother, Zubaida, to arbitrate the succession and champion his cause as Aisha had done two centuries before. Zubaida refused to do so, and al-Amin retired in despair to Baghdad. In 813, Tahir took Baghdad, and al-Amin was beheaded.