Abbasid calculations proved correct, as the two brothers spent much time quarrelling with each other. In 841, for instance, Bagrat had the Armenian bishops depose the Catholicos of Armenia, John IV, but he was promptly re-installed in his see by Smbat with the assistance of the other princes. Nevertheless, the Armenian princes were able to use the Caliphate's preoccupation with the Khurramite rebellion of Babak Khorramdin to achieve a significant degree of autonomy during this period. Smbat, who had spent time at the caliphal court as a hostage, was more circumspect about openly challenging Arab power than his brother, but both were ultimately too weak to seriously threaten Abbasid predominance for the time being. Thus Bagrat participated in the great campaign of Caliph al-Mu'tasim against the Byzantine Empire in 838, and even fought in the Battle of Dazimon against Emperor Theophilos. In 841, on the other hand, under the leadership of the "sparapet" Smbat, the Armenians revolted against the appointment as caliphal governor of Khalid ibn Yazid al-Shaybani, who in his previous tenures had become enormously unpopular among both the Christian and the Arab princes of the country. The rebels achieved his recall by the Caliph and his replacement with the weaker and more pliant Ali ibn Husayn, to whom the Armenians not only refused to hand over the expected taxes, but whom they promptly blockaded in his capital, Bardaa.