born Nov. 7, 639, Constantinople
died Sept. 15, 668, Syracuse, Sicily
Byzantine emperor (641–68).
His reign saw the loss of Byzantium's southern and eastern provinces to the Arabs. Muslim Arabs took Egypt (642), invaded Armenia (647), and defeated Constans at sea in 655. A civil war among Arabs prevented them from attacking Constantinople, and he secured a nonaggression treaty with Syria (659). Within the empire he tried to force unity on the church, forbidding debate on divisive theological questions and exiling the pope when he objected (653). He made his son coemperor (654) and then aroused public outrage by ordering the murder of his own brother (660). He left Constantinople (663) and settled in Sicily, where he was assassinated.