Constantius II, full name Flavius Julius Constantius (AD 317-361), Roman Emperor, 337-361. He was the third son of Constantine the Great, whose will left his empire to his sons Constantine II, Constantius II, and Constans as Augusti, with his nephews Dalmatius and Hannibalianus as Caesar and Nobilissimus respectively. On their accession, 337, Constantius is said to have allowed the murder of Dalmatius and Hannibalianus. Thrace, Macedonia, Greece, the Asiatic provinces, and Egypt were allotted to him. He had been Caesar under Constantine I as early as 333.
Throughout his reign he was at war with the Persians, and often defeated by them, notably in 348. When in 350 the revolt of Magnentius resulted in the death of Constans I, Constantius defeated the former at Mursa on River Drave, 351, and in Gaul, 353, becoming master of the whole Empire. In 355 Constantius made his cousin, the apostate Julian, Caesar and commander in Gaul. In 357 he visited Rome for the first time. He favoured the Arians, and banished the Orthodox bishops. When Julian was proclaimed Emperor by his troops in Gaul, Constantius moved west to oppose him, but died near Tarsus in Cilicia.