Al-Mutasim (794-842), eighth Abbasid caliph, a younger son of Harun ar-Rashid. Succeeding his brother al-Ma'mun in 833, al-Mutasim 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, al-Mutasim invaded Asia Minor, defeated Theophilus and destroyed the fortresses of Ancyra (Ankara) and Amorium.