Constantine IV, Byzantine emperor

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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