Al-Ma’mun was born on 13 September 786 AD in Baghdad. His father was Harun al-Rashid, who was an Abbasid caliph. He was a member of the third dynasty of the descendants of Muhammad who were caliphs of Islam. While his father ruled, al-Ma’mun was the governor of Khorasan. After al-Ma’mun was born his father had another son named al-Amin. Before his father died he announced that al-Amin would be the new caliph and al-Ma’mun would continue to be the governor of Khorasan even though al-Ma’mun was older. When Harun al-Rashid died and al-Amin became the caliph, he took away al-Ma’mun's ability to inherit the throne after he died. The two brothers went to war against each other. Al-Ma’mun defeated al-Amin. He cut off al-Amin's head. Since al-Amin had no other heir to the throne, al-Ma’mun became the new caliph.