ʿUmar himself probably became emir of Malatya in the 830s, and first appears in 838, in the Amorium campaign of the Caliph al-Mu'tasim (r. 833–842). The campaign was waged in retaliation for a large-scale raid in the previous year by the Byzantine emperor Theophilos (r. 829–842) against the Arab border emirates, including Malatya, whose territory was laid waste and depopulated. During the Amorium campaign, ʿUmar took part with his men in the great Arab victory over Theophilos himself at the Battle of Dazimon in July 838. In the 840s, he provided refuge to the surviving members of the Paulicians, who were fleeing persecution in Byzantium, and allocated them the area around the fortresses of Tephrike, Amara and Argaoun. The Paulician leader Karbeas turned this into a separate Paulician principality, allied with ‘Umar and launching frequent expeditions against Byzantium, either in conjunction with ʿUmar or independently. In 844, ʿUmar's forces participated in a major raid that inflicted a heavy defeat upon an army led by the Byzantine chief minister, Theoktistos, at the Battle of Mauropotamos. In the late 840s, he was also engaged in warfare against a neighbouring Armenian lord named Skleros, whom he finally vanquished after a protracted and bloody conflict.