Al-Mamun (813-833 AD) ascended to the Abbasid throne over the dead body of his brother al-Amin. Their father, Caliph Harun al-Rashid, had divided most of the empire among the two brothers. In this way al-Rashid tried to prevent squabbles among them. Not long after their father's death, however, war broke out among the brothers. This coin was struck in the city of Herat in Khorasan, the stronghold of al-Mamun. The Khorasan army played a decisive role in the war between the two brothers. Thanks to its superior art of warfare it defeated al-Amin's Arabic and Syrian troops and conquered the whole of Iran. Then it laid siege on Baghdad, where the caliph sat. When the city was finally taken, al-Amin was imprisoned and executed. Al-Mamun ascended to the throne.