(Constans Pogonatus), 630–68, Byzantine emperor (641–68), son and successor of Constantine III and grandson of Heraclius I. Early in his reign Armenia and Asia Minor were invaded by the Muslims, who challenged Byzantine supremacy at sea, took Cyprus, and threatened Sicily and Constantinople. An able and vigorous ruler, he sought to end the religious controversy centering about Monotheletism by issuing a decree (648) forbidding its discussion. This involved him in conflict with Pope Martin I, whom he finally had arrested and banished. Constans campaigned (658) in the Balkans against the Slavs, and in 662 he moved to Italy, with the purpose of establishing his capital at Rome, but fought with little result against the Lombards and finally settled (663) at Syracuse. From there he directed a successful resistance to the Muslims. Constans extended the administrative reorganization of the empire begun by Heraclius. Assassinated, he was succeeded by his son, Constantine IV.