Of August, A.D. 317. He was declared Caesar, and Princeps Juventutis on the 8th of November, 323; and being created Consul in 326, he was entrusted by his father, at the age of 15 years, with the administration of affairs in Gaul. In the partition which that emperor made of his dominion in 335, Asia, Syria, and Egypt were assigned to Constantius. At the death of his celebrated father in 337, he immediately quitted the eastern provinces of which he was holding the government, and hastening to Constantinople, was there acknowledged as Augustus, at the same time with his brothers (Constantine jun. and Constans). In the arrangements afterwards made, he kept the East for himself. The army had already proclaimed their determination, that none should reign but the sons of Constantine; thus excluding Delmatius and Hanniballianus from the sovereignty of those provinces which their uncle had assigned to them. So far from evincing any displeasure at this instance of military dictation, it was he who, according to general belief, instigated the soldiers to massacre the male descendants of his grand-father Constantius Chlorus, with the exception only of Gallus and Julianus. After implicating himself in this atrocious act of perfidy and bloodshed, Constantius met his brothers at Sirmium, in 337, for the purpose of dividing the empire anew; and three youths of twenty-one, twenty, and seventeen years of age, partitioned out between themselves the government of the Roman world. But scarcely had Constantius taken possession of his share of the spoil (which share comprised Thrace, Macedonia, Greece, the Asiatic provinces, and Egypt), when he found himself engaged in a war with Sapor the Second, King of Persia, a war chiefly waged in Mesopotamia and the Syrian frontier, and which, with brief intervals, continued during the whole of this prince 's reign. He was accustomed to pass the winters at Antioch, and to employ the summers in ravaging the Persian territories. In these campaigns Constantius fought the enemy, sometimes with glory, but frequently with dishonor. Amongst the many battles which turned to his disadvantage was that of Siogara, in 348, when he commanded in person; and, after having been victorious during the day, he was defeated in the succeeding night, with immense loss to his army.
In 350, having left Persia to oppose Magnentius, who, after causing Constans to be murdered, had succeeded in his attempt to become master of the western empire, Constantius was for some time under the necessity of tolerating a colleagueship with Vetranio, who commanded the Illyrian legions, and who, like Magnentius, had assumed the purple, and the title of Augustus. Constantius at length, however, having compelled Vetranio to renounce his imperial rank and government, proceeded, in A.D. 351, in search of Magnentius, whom he defeated at Mursa, now Essek, a town on the banks of the Drave, in Hungary.----Magnentius fled into Gaul, and being again routed in two consecutive engagements by the armies of the emperor, this usurper put an end to his own life at Lyon, A.D. 353; his brother Decentius following his tragical example. Constantius thus became master of the whole west. Meanwhile he had given the title of Caesar to his cousin Gallus; but the crimes to which that young prince abandoned himself, were such that, by the emperor 's order, he was beheaded, after a reign of about four years. (See CONSTANTIUS GALLUS.)----On the sixth of November, 355, Constantius conferred the title of Caesar on Julian, the brother of Gallus, to whom he gave his sister Helena in marriage, investing him, at the same time, with the government of the Gauls, Spain, and Britain.
Having obtained peace for the empire, Constantius made preparations to visit Rome, which he had not yet seen. He made his entry there on the 28th of April, 357, in the habiliments of a Triumpher, although no captives followed his chariot, and he was surrounded by none but his courtiers and a detachment of his troops.----Astonished and enraptured at the magnificence of the city, he ordered the great obelisk, which his father had caused to be brought from Heliopolis, in Egypt, and which was remaining at Alexandria, to be transported to Rome, where it was erected in the Circus Maximus. Returned to Mesopotamia, in 359, to meet the invading armies of Sapor, he received the tidings that Julian had been proclaimed Emperor of the West. This event induced Constantius to retrace his steps: and in 360, having re-assembled nearly all the legions of the East, he marched with them to encounter his relation and rival. But agitation and excitement, added to the fatigue of the expedition, threw him into a fever. He halted at Mopsocrene, a small town situated at the foot of Mount Taurus; and after having declared Julian his successor and sole master of the empire, he died on the third of November, A.D. 361, in the 25th year of his reign, and 45th of his age.----Julian caused his remains to be conveyed to Constantinople; received the body at the gates of that city, amidst his soldiers under arms; and interred it in the tomb of Constantine the Great.