Abū Jaʿfar Abdullāh al-Maʾmūn ibn Hārūn al-Rashīd (Arabic: ابوجعفر عبدالله المأمون‎‎; September 786 – 9 August 833) was the seventh Abbasid caliph, who reigned from 813 until his death in 833. He succeeded his brother al-Amin after a civil war, and was also known for his role in the Mu'tazilism controversy, and the resumption of large-scale warfare with the Byzantine Empire.

The future al-Ma'mun was born in Baghdad on the night of the 13 to 14 September 786 CE to Harun al-Rashid and his concubine Marajil, from Badghis. On the same night, which later became known as the "night of the three caliphs", his uncle al-Hadi died and was succeeded by Ma'mun's father, Harun al-Rashid, as ruler of the Abbasid Caliphate.

In 802 Harun al-Rashid, father of al-Maʾmūn and al-Amin, ordered that al-Amin succeed him, and al-Ma'mun serve as governor of Khurasan and as caliph after the death of al-Amin. In the last days of Harun's life his health was declining and saw in a dream Musa ibn Jafar sitting in a chamber praying and crying, which made Harun remember how hard he had struggled to establish his own caliphate. He knew the personalities of both his sons and decided that for the good of the Abbasid dynasty, al-Maʾmūn should be caliph after his death, which he confided to a group of his courtiers. One of the courtiers, Fadl ibn Rabi' did not abide by Harun's last wishes and convinced many in the lands of Islam that Harun's wishes had not changed. Later the other three courtiers of Harun who had sworn loyalty to Harun by supporting al-Maʾmūn, namely 'Isa Jarudi, Abu Yunus, and Ibn Abi 'Umran found loopholes in Fadl's arguments, and Fazl admitted Harun had appointed al-Maʾmūn after him, but, he argued, since Harun was not in his right mind, his decision should not be acted upon. Al-Maʾmūn was reportedly the older of the two brothers, but his mother was a Persian woman while al-Amin's mother was a member of the reigning Abbasid family. After al-Rashid's death in 809, the relationship between the two brothers deteriorated. In response to al-Ma'mun's moves toward independence, al-Amin declared his own son Musa to be his heir. This violation of al-Rashid's testament led to a succession struggle. al-Amin assembled a massive army at Baghdad with 'Isa ibn Mahan at its head in 811 and invaded Khorasan, but al-Maʾmūn's general Tahir ibn al-Husayn (d. 822) destroyed the army and invaded Iraq, laying siege to Baghdad in 812. In 813 Baghdad fell, al-Amin was beheaded, and al-Maʾmūn became the undisputed Caliph.

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