Abdallah al-Mamun (786-833) was the seventh caliph, or ruler of the Islamic Empire, of the Abbasid dynasty. During his reign he encouraged the study of Greek thought, and the balance of power within the caliphate shifted from the Arabs to the non-Arabs.

Al-Mamun was the son of Harun al-Rashid and a Persian slave girl, Marajil. Born about May 786, he was slightly older than his half brother Amin, son of the Arab Zubayda, but Amin was first heir to the caliphate and al-Mamun second. Al-Mamun, however, became governor of the eastern provinces, with Fadl ibn-Sahl as vizier. When Harun died in March 809, al-Mamun recognized Amin as caliph but kept his position in the eastern provinces. Tension between the half brothers led to war in 811, which ended in September 813 with the fall of Baghdad and the death of Amin.

Al-Mamun became caliph but continued to reside in the East despite...