Constantius II (Latin: Flavius Julius Constantius; Ancient Greek: Κωνστάντιος, romanized: Kōnstántios; 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, court intrigues, and usurpations. His religious policies inflamed domestic conflicts that would continue after his death.
Constantius was a son of Constantine the Great, who elevated him to the imperial rank of Caesar on 8 November 324 and after whose death Constantius became Augustus together with his brothers, Constantine II and Constans on 9 September 337. He promptly oversaw the massacre of his father-in-law, an uncle, and several cousins, consolidating his hold on power. The brothers divided the empire among themselves, with Constantius receiving Greece, Thrace, the Asian provinces, and Egypt in the east. For the following decade a costly and inconclusive war against Persia took most of Constantius's time and attention. In the meantime, his brothers Constantine and Constans warred over the western provinces of the empire, leaving the former dead in 340 and the latter as sole ruler of the west. The two remaining brothers maintained an uneasy peace with each other until, in 350, Constans was overthrown and assassinated 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 died by 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's younger half-brother Julian, to the rank of Caesar.
As emperor, Constantius promoted Arianism, banned pagan sacrifices, and issued laws against Jews. His military campaigns against Germanic and Iranian 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 travelled to the east in 360 to restore stability after the loss of several border fortresses. However, Julian claimed the rank of Augustus in 360, leading to war between the two after Constantius's attempts to persuade Julian to back down failed. No battle was fought, as Constantius became ill and died of fever on 3 November 361 in Mopsuestia, allegedly naming Julian as his rightful successor before his death.
Flavius Julius Constantius was born in 317 at Sirmium, Pannonia, now Serbia. 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 8 November 324, at the age of 7. 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 (336). 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.
In early 337, Constantius hurried to Constantinople after receiving news that his father was near death. After Constantine died, Constantius buried him with lavish ceremony in the Church of the Holy Apostles. Soon after his father's death, the army massacred his relatives descended from the marriage of his paternal grandfather Constantius Chlorus to Flavia Maximiana Theodora, though the details are unclear. Two of Constantius's uncles (Julius Constantius and Flavius Dalmatius) and seven of his cousins were killed, including Hannibalianus and Dalmatius, rulers of Pontus and Moesia respectively, leaving Constantius, his two brothers Constantine II and Constans, and three cousins Gallus, Julian and Nepotianus as the only surviving male relatives of Constantine the Great. While the “official version” was that Constantius's relatives were merely the victims of a mutinous army, Ammianus Marcellinus, Zosimus, Libanius, Athanasius and Julian all blamed Constantius for the event. Burgess considered the latter version to be “consistent with all the evidence”, pointing to multiple factors that he believed lined up with the massacre being a planned attack rather than a spontaneous mutiny:—the lack of high-profile punishments as a response; the sparing of all women; the attempted damnatio memoriae on the deceased; and the exile of the survivors Gallus and Julian.
Soon after, Constantius met his brothers in Pannonia at Sirmium to formalize the partition of the empire. Constantius received the eastern provinces, including Constantinople, Thrace, Asia Minor, Syria, Egypt, and Cyrenaica; Constantine received Britannia, Gaul, Hispania, and Mauretania; and Constans, initially under the supervision of Constantine II, received Italy, Africa, Illyricum, Pannonia, Macedonia, and Achaea.
Constantius then hurried east to Antioch to resume the war with Persia. While Constantius was away from the eastern frontier in early 337, King Shapur II assembled a large army, which included war elephants, and launched an attack on Roman territory, laying waste to Mesopotamia and putting the city of Nisibis under siege. Despite initial success, Shapur lifted his siege after his army missed an opportunity to exploit a collapsed wall. When Constantius learned of Shapur's withdrawal from Roman territory, he prepared his army for a counter-attack.
Constantius repeatedly defended the eastern border against invasions by the Sassanid Empire under Shapur. These conflicts were mainly limited to Sassanid sieges of the major fortresses of Roman Mesopotamia, including Nisibis (Nusaybin), Singara, and Amida (Diyarbakir). Although Shapur seems to have been victorious in most of these confrontations, the Sassanids were able to achieve little. However, the Romans won a decisive victory at the battle of Narasara, killing Shapur's brother, Narses. Ultimately, Constantius was able to push back the invasion, and Shapur failed to make any significant gains.
Meanwhile, Constantine II desired to retain control of Constans's realm, leading the brothers into open conflict. Constantine was killed in 340 near Aquileia during an ambush. As a result, Constans took control of his deceased brother's realms and became sole ruler of the Western two-thirds of the empire. This division lasted until January 350, when Constans was assassinated by forces loyal to the usurper Magnentius.
Constantius was determined to march west to fight the usurper. However, feeling that the east still required some sort of imperial presence, he elevated his cousin Constantius Gallus to caesar of the eastern provinces. As an extra measure to ensure the loyalty of his cousin, he married the elder of his two sisters, Constantina, to him.
Before facing Magnentius, Constantius first came to terms with Vetranio, a loyal general in Illyricum who had recently been acclaimed emperor by his soldiers. Vetranio immediately sent letters to Constantius pledging his loyalty, which Constantius may have accepted simply in order to stop Magnentius from gaining more support. These events may have been spurred by the action of Constantina, who had since traveled east to marry Gallus. Constantius subsequently sent Vetranio the imperial diadem and acknowledged the general's new position as augustus. However, when Constantius arrived, Vetranio willingly resigned his position and accepted Constantius's offer of a comfortable retirement in Bithynia.