CONSTANTINE IV., Pogonatus (the "bearded"), son of Constans II., was emperor from 668 to 685. After his accession he crushed an Armenian usurper in Sicily. For six years (672-77) the Arabs under the caliph Moawiya (see CALIPHATE) besieged Constanti nople, but were obliged at last to make peace and agree to pay tribute for 3o years. The attacks of the Slays and Avars upon Thessalonica were repulsed. But Constantine was unable to pre vent the Bulgars from crossing the Danube and settling in the dis trict where their name still survives. The Bulgarian kingdom was established under its first king Isperich in 679. The tribute paid by the Arabs was used to purchase the good will of the new settlers. Constantine summoned the sixth ecumenical council at Constantinople, which sat from Nov. 7, 68o, to Sept. 16, 681.