Constantius II (Latin: Flavius Julius Constantius Augustus; Greek: Κωνστάντιος; 7 August 317 – 3 November 361) was Roman Emperor from 337 to 361. His reign saw constant warfare on the borders against the Sasanian Empire and Germanic peoples, while internally the Roman Empire went through repeated civil wars and usurpations. His religious policies inflamed domestic conflicts that would continue after his death.
The second son of Constantine I and Fausta, Constantius was made Caesar by his father in 324. He led the Roman army in war against the Sasanian Empire in 336. A year later, Constantine I died, and Constantius became Augustus with his brothers Constantine II and Constans. He promptly oversaw the massacre of eight of his relatives, consolidating his hold on power. The brothers divided the empire among themselves, with Constantius receiving the eastern provinces. In 340, his brothers Constantine and Constans clashed over the western provinces of the empire. The resulting conflict left Constantine dead and Constans as ruler of the west. The war against the Sasanians continued, with Constantius losing a major battle at Singara in 344. Constans was overthrown and assassinated in 350 by the usurper Magnentius.
Unwilling to accept Magnentius as co-ruler, Constantius waged a civil war against the usurper, defeating him at the battles of Mursa Major in 351 and Mons Seleucus in 353. Magnentius committed suicide after the latter battle, leaving Constantius as sole ruler of the empire. In 351, Constantius elevated his cousin Constantius Gallus to the subordinate rank of Caesar to rule in the east, but had him executed three years later after receiving scathing reports of his violent and corrupt nature. Shortly thereafter, in 355, Constantius promoted his last surviving cousin, Gallus' younger half-brother Julian, to the rank of Caesar.
As emperor, Constantius promoted Arian Christianity, persecuted pagans by banning sacrifices and closing pagan temples and issued laws discriminating against Jews. His military campaigns against Germanic tribes were successful: he defeated the Alamanni in 354 and campaigned across the Danube against the Quadi and Sarmatians in 357. The war against the Sasanians, which had been in a lull since 350, erupted with renewed intensity in 359 and Constantius traveled to the east in 360 to restore stability after the loss of several border fortresses to the Sasanians. However, Julian claimed the rank of Augustus in 360, leading to war between the two after Constantius' attempts to convince Julian to back down failed. No battle was fought, as Constantius became ill and died of fever on 3 November 361 in Mopsuestia, naming Julian as his rightful successor before his death.
Constantius was born in 317 at Sirmium, Pannonia. He was the third son of Constantine the Great, and second by his second wife Fausta, the daughter of Maximian. Constantius was made Caesar by his father on 13 November 324.
In 336, religious unrest in Armenia and tense relations between Constantine and king Shapur II caused war to break out between Rome and Sassanid Persia. Though he made initial preparations for the war, Constantine fell ill and sent Constantius east to take command of the eastern frontier. Before Constantius arrived, the Persian general Narses, who was possibly the king's brother, overran Mesopotamia and captured Amida. Constantius promptly attacked Narses, and after suffering minor setbacks defeated and killed Narses at the Battle of Narasara. Constantius captured Amida and initiated a major refortification of the city, enhancing the city's circuit walls and constructing large towers. He also built a new stronghold in the hinterland nearby, naming it Antinopolis.