Al-Mu'tasim's reign was marked by continuous warfare. The two major internal campaigns of the reign were against the long-running Khurramite uprising of Babak Khorramdin in Adharbayjan, which was suppressed by al-Afshin in 835–837, and against Mazyar, the autonomous ruler of Tabaristan, who had clashed with the Tahirid governor of Khorasan and risen up in revolt. While his generals led the fight against internal rebellions, al-Mu'tasim himself led the sole major external campaign of the period, in 838 against the Byzantine Empire. His armies defeated Emperor Theophilos and sacked the city of Amorium. The Amorium campaign was widely celebrated, and became a cornerstone of caliphal propaganda, cementing al-Mu'tasim's reputation as a warrior-caliph.