Al-Mu'tasim (796–842) was the eighth Abbasid caliph. A younger son of Caliph Harun al-Rashid, he rose to prominence as a key lieutenant of his brother Caliph al-Ma'mun after forming a private army composed predominantly of Turkish slave-soldiers. When al-Ma'mun died on campaign in August 833, al-Mu'tasim succeeded him and continued many of his policies, including support for Mu'tazilism. The traditional Arab and Iranian elites were weakened in favour of a new elite drawn from among the Turks, while the government was centralized around the caliphal court and a new capital founded to house it at Samarra. Al-Mu'tasim also achieved lasting fame as a warrior-caliph by sacking the Byzantine city of Amorium in 838. The rise of the Turks would eventually lead to factional strife and the collapse of Abbasid power in the mid-10th century, but the slave-soldier system inaugurated by al-Mu'tasim would be widely adopted throughout the Muslim world for centuries to come.