al-Maʾmun

Abu al-Abbas Abd Allah ibn Harun (Arabic: أبو العباس عبد الله بن هارون الرشيد, romanized: Abū al-ʿAbbās ʿAbd Allāh ibn Hārūn ar-Rashīd; 14 September 786 – 9 August 833), better known by his regnal name al-Ma'mun (Arabic: المأمون, lit. 'the Reliable'), was the seventh Abbasid caliph, who reigned from 813 until his death in 833. His leadership was marked by the power and prosperity of the Abbasid Caliphate; al-Ma'mun promoted the Graeco-Arabic translation movement, the flowering of learning and the sciences in Baghdad, and the publishing of al-Khwarizmi's book now known as "Algebra", making al-Ma'mun one of the most important caliphs in the Islamic Golden Age. He is also known as a proponent of the rational Islamic theology of Mu'tazilism.

Al-Ma'mun succeeded his half-brother al-Amin after a civil war, and much of his reign was spent on peace campaigns. His strong support for Mu'tazilism led him to imprison a Sunni Imam, Ahmad ibn Hanbal in an event that became known as mihna. Al-Ma'mun's foreign policy was due to his decision to continue war and diplomatic relations with the Byzantine Empire, the tension between conflict or diplomacy varying during his military campaigns.