MU'TASIM, AL-

born 794 died Jan. 5, 842 eighth 'Abbasid caliph, a younger son of Harun ar-Rashid. Succeeding his brother al-Ma'mun in 833, al-Mu'tasim was the first caliph to employ the Turkish mercenaries who later came to dominate the 'Abbasid dynasty. In 837 he crushed a revolt of Persian schismatics led by the rebel Babak, who was cooperating with the Greeks. After the Byzantine emperor Theophilus had laid waste the Muslim town of Zibatra (known to the Byzantines as Sozopetra), al-Mu'tasim invaded Asia Minor, defeated Theophilus, and destroyed the fortresses of Ancyra (Ankara) and Amorium (August 838).