Constantine IV, c.652–685, Byzantine emperor (668–85), son and successor of Constans II. He defended Constantinople against the annual naval attacks of the Muslims, who finally withdrew in 678; Greek fire was a conspicuous weapon in the defense. Severely defeated (679) by the Bulgars, Constantine ceded them territory S of the Danube, where they founded a kingdom. In 680 he summoned the Third Council of Constantinople, which briefly reestablished peace between the Eastern and Western churches by condemning Monotheletism. Constantine was succeeded by his son, Justinian II.
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