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Constantius II (317–361) was Roman emperor from 337 to 361. He was the son of Constantine I the Great and Fausta. In 337, Constantine died and Constantius II became co-emperor with his three brothers, Constantine II, Constans, and Dalmatius. After Constantine II and Constans were killed in battle in 340, Constantius II became sole emperor. He ruled the Roman Empire during a period of political and military instability.