Constanti'nusIv., Fla'vius> or Fla'vius>Pogona'tus

surnamed POGONA'TUS or BARBA'TUS, emperor of the East, A. D. 668-685, the eldest son of Constans II., succeeded his father in 668. Constans having lost his life by assassination at Syracuse, his murderers, who seemed to have had great power, and who were assisted by the Greek army stationed in Sicily, chose as emperor one Mizizus, Mecentius, or Mezzetius, an Armenian. Constantine fitted out an expedition against the usurper, quelled the rebellion in 669, and put Mizizus to death. After a short stay at Syracuse, Constantine sailed back to Constantinople, carrying with him the body of his father; but no sooner was he gone, than an Arabic fleet, perhaps invited thither by the rebels, appeared off Syracuse. The place was taken by surprise and partly destroyed, and the riches and statues, the plunder of Rome, collected there by Constans, were carried by the Arabs to Alexandria. The Greek troops in Asia revolted soon after the return of the emperor. They would be governed by a " Trinity," and not by a sole sovereign, and demanded that Constantine should divide his authority with his two brothers, Heraclius and Tiberius, who had the title but not the power of Augusti. This rebellion was likewise soon quelled, and Constantine pardoned both his brothers. At the same time, an Arabic army commanded by Ukbah and Dinár invaded the remaining part of the Greek dominions in Africa (Mauretania), penetrated as far as the shores of the Atlantic, and ravaged the country so fearfully, that both the Greek and Berber inhabitants rose in despair, and, under the command of a native chief named Kussileh, surprised the Moslems, and killed nearly all of them. This however was no advantage to the emperor, since Kussileh succeeded in seizing the supreme power in that country.

In 671 the Arabs equipped a powerful fleet with the intention of laying siege to Constantinople. They conquered Smyrna and nearly all the islands of the Grecian archipelago, and began the blockade of Constantinople in the spring of 672 ; but, after a protracted siege of five months,were compelled to sail back, after sustaining immense losses from the Greek fire, which had just been invented by Callinicus, a native of Heliopolis in Syria, and was first employed in that siege. Yezíd, the son of the khalif Mú'awiyah, who commanded the Arabic forces, returned in the following spring, and, during a period of seven years, regularly appeared before Constantinople in the spring, and sailed to his winter-quarters in the autumn, but was not able to take the city. During the last siege, in 679, the Arabic fleet lost so many ships by the Greek fire, that Yezíd was compelled to make a hasty retreat, and not having a sufficient number of ships for his numerous forces, despatched a body of 30,000 men by land for Syria, while he embarked the rest on board his fleet. But his fleet was destroyed by a storm, and the land army was overtaken and cut to pieces by a Greek army commanded by Florus, Petronas, and Cyprianus. This unfortunate campaign, and the war at the same time with the Maronites or Druses of Mount Lebanon, pressed so heavily upon the khalif Mú'awiyah, that, wishing for peace, he signed the conditions offered him by Constantine, and he thus became liable, for the period of thirty years, to an annual tribute of 3000 pounds of gold accompanied by rich presents of slaves and horses. By this glorious peace the authority of the Greek emperor rose to such a height, that all the minor powers of Asia sought his protection. But his name was less dreaded in Europe, for he was compelled by the Bulgarians to cede to them that country south of the Danube which is still called Bulgaria.

In 680 Constantine assembled the sixth general council at Constantinople, by which the Monothelists were condemned and peace was restored to the church. In 681 the emperor's brothers, Heraclius and Tiberius, were both deprived of their dignity of Augustus, which title Constantine conferred upon his son Justinian. We know almost nothing of the last five years of the reign of Constantine : he died in the month of September, 685, and was succeeded by his son, Justinian II.

Besides the wars which signalized the reign of Constantine IV., there is an event not less remarkable, which most probably took place during the same period. We allude to the new division of the empire, which had hitherto been administered according to the ancient system, so that, for instance, all the Asiatic dominions were ruled by a civil governor or proconsul, and the whole army stationed in that part of the empire had likewise but one chief commander, the praefect of Asia. The constant incursions of the Arabs required the presence of different moveable corps stationed in the frontier provinces, the commanders of which were independent of one another : these bodies were called themata (θέματα), from thema (πέμα), a position. This name was afterwards given to the districts in which such corps were stationed, and its use became so general, that at last the whole empire was divided into twenty-nine themata, seventeen of which were in the eastern and southern or Asiatic part of the empire, and twelve in the northern and western parts, from the Cimmerian Bosporus to Sicily. This important change in the administration of the empire took place in the latter years of the reign of Heraclius, or in the reign of Constantine IV., that is, from about 635 to 685. But although we do not precisely know the year, there are many reasons for believing that Constantine IV. was the originator of that plan. [CONSTANTINUS VII.] (Cedren. p. 436, &c., ed. Paris; Zonar. vol. ii. p. 89, &c., ed. Paris; Glycas, p. 278, &c., ed. Paris; Theophan. p. 289, &c., ed. Paris; Paulus Diacon. De Gestis Longobard. 5.30.)

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