CONSTANTIUS, FLAVIUS JULIUS em peror of the East 337-361, was the son of Constantine the Great and Fausta, and was born at Sirmium in Pannonia on Aug. 6, 317. When the three sons of Constantine were made Caesars, Constantius was given the Eastern provinces. Constantine's death in 337 was followed by a wholesale massacre of the rest of the family by the soldiers, to secure the succession of the desig nated sons, in which only Julian and Gallus escaped. A conference in Pannonia followed, at which it was arranged that Constantine II. took the West, Constantius the East (Thrace, Macedonia, Greece, Asia and Egypt) and Constans Italy and Illyricum. Con stantius was immediately plunged into war with Shapur (q.v.), which lasted throughout the reign. The early campaigns are obscure and seem to have been indecisive, but he won a fairly complete victory at Singara in 344 or 348. In 35o occurred the revolt of the West and the usurpation of Magnentius. Constantius refused to abandon the war with Shapur, but took advantage of the insurrection of Vetranio to use him against Magnentius. Then Shapur's attention was diverted by the Massagetae; Constantius at once sent Gallus to Syria with the title of Caesar, and turned west to join Vetranio. Together they defeated Magnentius at a great battle at Mursa on the Drave on Sept. 28, 351. The losses on both sides were enormous, and the Roman forces were permanently weakened. The eastern and western empires were thus again united under Constantius, a monarchy of the oriental type.
In 355 the critical situation in Gaul led to the appointment of Julian as commander there, which was entirely successful (for these campaigns see JULIAN) . Constantius visited Rome in 357, where he set up the obelisk from Heliopolis (now the Lateran obelisk) in the Circus Maximus, and moved the altar of Victory from the Curia, marking the end of official paganism. In 359 he went back to Asia to meet Shapur. His order that the Gallic troops should come East after Julian's victories in Gaul led to their revolt and proclamation of Julian as emperor. Constantius was unwilling to leave the war with Shapur, and both sides made at tempts at compromise. Eventually Constantius started to march West, but he died on the way, near Tarsus in Cilicia, on Nov. 3, 361.
Constantius took his responsibilities as emperor very seriously. On two critical occasions he sacrificed his own interests to the claims of the empire, and stayed on the eastern frontier when his fortunes were at stake in Gaul. His fault was his dependence on the inner circle of palace officials, due to the unapproachable nature of the oriental monarchy founded by Diocletian. In the religious quarrels of the time he supported the anti-Athanasian party, was forced by Constans to take Athanasius back in 346, but had him accused at Arles after his victory over Magnentius in 351.
See Amm. Marc., xiv.–xxi.; Zosimus, iii.; Agathion, iv.; Zonaras, Cambridge Mediaeval History, vol. i.