Al-Mamun (786–833), the seventh Abbasid caliph, is conventionally dated as ruling from 813 to 833. His reign combined civil war, government from Khurasan and Baghdad, support for scholarship, and a late attempt to enforce a theological doctrine through the mihna.
Named Abd Allah, al-Mamun was a son of Harun al-Rashid. The succession arrangements of 802 placed al-Amin first and al-Mamun second, while giving the latter a base in Khurasan. After Harun’s death in 809, conflicts over succession, troops and revenues led to civil war in 811.
Al-Mamun governed from Merv while commanders including Tahir advanced westwards. Baghdad fell in 813 and al-Amin was killed after surrendering. Victory established al-Mamun as caliph, but did not immediately bring him back to the capital or end provincial resistance.
In 817 al-Mamun designated the Shiite imam Ali al-Rida as heir. Opposition helped produce a rival caliphate under Ibrahim in Baghdad. Al-Rida died in 818, and al-Mamun entered Baghdad in 819. Provincial government still depended heavily on military commanders and local rulers.
Al-Khwarizmi’s authorial preface to his algebra praises the caliph’s support for learning. It identifies practical problems of inheritance, commerce, land measurement and canal digging as reasons for writing. This is a patronage statement by its author, but also concrete evidence of the purposes assigned to mathematical work.
In the final months of his life in 833, al-Mamun initiated the mihna, an examination intended to enforce belief that the Quran was created. The policy asserted the caliph’s authority over a central theological question. Scholarly patronage and religious coercion were both features of his government.
He died during a campaign in Cilicia, and his successor al-Mutasim continued the examination. Bosworth notes its enforcement in regions from Egypt to Iraq, while finding no firm evidence of a similarly substantial effect in Khurasan. Its reach therefore needs to be considered regionally as well as chronologically.