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.

Gold solidus of Constantius Gallus. A paternal cousin of Constantius, he was made Caesar by Constantius in 350 and was married to the Emperor's sister, Constantina. However, his mismanagement of the eastern provinces led to his downfall and death in 354.

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.[9] Soon after his father's death Constantius supposedly ordered a massacre of his relatives descended from the second marriage of his paternal grandfather Constantius Chlorus, though the details are unclear. Eutropius, writing between 350 and 370, states that Constantius merely sanctioned “the act, rather than commanding it”. The massacre killed two of Constantius' uncles and six of his cousins,[13] including Hannibalianus and Dalmatius, rulers of Pontus and Moesia respectively. The massacre left Constantius, his older brother Constantine II, his younger brother Constans, and three cousins Gallus, Julian and Nepotianus as the only surviving male relatives of Constantine the Great.

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.[18] 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 aggressive 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' 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 350, when Constans was assassinated by forces loyal to the usurper Magnentius.

As the only surviving son of Constantine the Great, Constantius felt that the position of emperor was his alone,[23] and he determined to march west to fight the usurper, Magnentius. 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’ offer of a comfortable retirement in Bithynia.

In 351, Constantius clashed with Magnentius in Pannonia with a large army. The ensuing Battle of Mursa Major was one of the largest and bloodiest battles ever between two Roman armies. The result was a victory for Constantius, but a costly one. Magnentius survived the battle and, determined to fight on, withdrew into northern Italy. Rather than pursuing his opponent, however, Constantius turned his attention to securing the Danubian border, where he spent the early months of 352 campaigning against the Sarmatians along the middle Danube. After achieving his aims, Constantius advanced on Magnentius in Italy. This action led the cities of Italy to switch their allegiance to him and eject the usurper's garrisons. Again, Magnentius withdrew, this time to southern Gaul.

In 353, Constantius and Magnentius met for the final time at the Battle of Mons Seleucus in southern Gaul, and again Constantius emerged the victor. Magnentius, realizing the futility of continuing his position, committed suicide on 10 August 353.

Sole ruler of the empire

Constantius spent much of the rest of 353 and early 354 on campaign against the Alamanni on the Danube frontier. The campaign was successful and raiding by the Alamanni ceased temporarily. In the meantime, Constantius had been receiving disturbing reports regarding the actions of his cousin Gallus.[31] Possibly as a result of these reports, Constantius concluded a peace with the Alamanni and traveled to Mediolanum (Milan).

In Mediolanum, Constantius first summoned Ursicinus, Gallus’ magister equitum, for reasons that remain unclear.[33] Constantius then summoned Gallus and Constantina. Although Gallus and Constantina complied with the order at first, when Constantina died in Bithynia, Gallus began to hesitate. However, after some convincing by one of Constantius’ agents, Gallus continued his journey west, passing through Constantinople and Thrace to Poetovio (Ptuj) in Pannonia.

In Poetovio, Gallus was arrested by the soldiers of Constantius under the command of Barbatio. Gallus was then moved to Pola and interrogated. Gallus claimed that it was Constantina who was to blame for all the trouble while he was in charge of the eastern provinces.[39] This angered Constantius so greatly that he immediately ordered Gallus' execution. He soon changed his mind, however, and recanted the order. Unfortunately for Gallus, this second order was delayed by Eusebius, one of Constantius' eunuchs, and Gallus was executed.

Religious issues

Constantius II depicted in the Chronography of 354 dispensing largesse (a Renaissance copy of a Carolingian copy)

In spite of some of the edicts issued by Constantius, he never made any attempt to disband the various Roman priestly colleges or the Vestal Virgins, he never acted against the various pagan schools, and, at times, he actually made some effort to protect paganism. In fact, he even ordered the election of a priest for Africa. Also, he remained pontifex maximus and was deified by the Roman Senate after his death. His relative moderation toward paganism is reflected by the fact that it was over twenty years after his death, during the reign of Gratian, that any pagan senator protested his treatment of their religion.

Christianity

Although often considered an Arian, Constantius ultimately preferred a third, compromise version that lay somewhere in between Arianism and the Nicene Creed, retrospectively called Semi-Arianism.[46][47] During his reign he attempted to mold the Christian church to follow this compromise position, convening several Christian councils. The most notable of these were the Council of Rimini and its twin at Seleucia, which met in 359 and 360 respectively. "Unfortunately for his memory the theologians whose advice he took were ultimately discredited and the malcontents whom he pressed to conform emerged victorious," writes the historian A.H.M. Jones. "The great councils of 359–60 are therefore not reckoned ecumenical in the tradition of the church, and Constantius II is not remembered as a restorer of unity, but as a heretic who arbitrarily imposed his will on the church."

Judaism faced some severe restrictions under Constantius, who seems to have followed an anti-Jewish policy in line with that of his father.[57] Early in his reign, Constantius issued a double edict in concert with his brothers limiting the ownership of slaves by Jewish people[58] and banning marriages between Jews and Christian women.[58] A later edict issued by Constantius after becoming sole emperor decreed that a person who was proven to have converted from Christianity to Judaism would have all of his property confiscated by the state.[59] However, Constantius' actions in this regard may not have been so much to do with Jewish religion as with Jewish business—apparently, privately owned Jewish businesses were often in competition with state-owned businesses. As a result, Constantius may have sought to provide an advantage to state-owned businesses by limiting the skilled workers and slaves available to Jewish businesses.

On 11 August 355, the magister militum Claudius Silvanus revolted in Gaul. Silvanus had surrendered to Constantius after the Battle of Mursa Major. Constantius had made him magister militum in 353 with the purpose of blocking the German threats, a feat that Silvanus achieved by bribing the German tribes with the money he had collected. A plot organized by members of Constantius' court led the emperor to recall Silvanus. After Silvanus revolted, he received a letter from Constantius recalling him to Milan, but which made no reference to the revolt. Ursicinus, who was meant to replace Silvanus, bribed some troops, and Silvanus was killed.

Constantius realised that too many threats still faced the Empire, however, and he could not possibly handle all of them by himself. So on 6 November 355, he elevated his last remaining male relative, Julian, to the rank of Caesar. A few days later, Julian was married to Helena, the last surviving sister of Constantius.[62] Constantius soon sent Julian off to Gaul.

Constantius spent the next few years overseeing affairs in the western part of the empire primarily from his base at Mediolanum. In 357 he visited Rome for the only time in his life. The same year, he forced Sarmatian and Quadi invaders out of Pannonia and Moesia Inferior, then led a successful counter-attack across the Danube.

In the winter of 357–58, Constantius received ambassadors from Shapur II who demanded that Rome restore the lands surrendered by Narseh. Despite rejecting these terms, Constantius tried to avert war with the Sassanid Empire by sending two embassies to Shapur II. Shapur II nevertheless launched another invasion of Roman Mesopotamia. In 360, when news reached Constantius that Shapur II had destroyed Singara, and taken Kiphas (Hasankeyf), Amida,  and Ad Tigris (Cizre), he decided to travel east to face the re-emergent threat.

Usurpation of Julian and crises in the east

Missorium of Kerch depicting Constantius II on horseback with a spear. He is preceded by victory and accompanied by a guardsman.

In the meantime, Julian had won some victories against the Alamanni, who had once again invaded Roman Gaul. However, when Constantius requested reinforcements from Julian's army for the eastern campaign, the Gallic legions revolted and proclaimed Julian Augustus.

On account of the immediate Sassanid threat, Constantius was unable to directly respond to his cousin's usurpation, other than by sending missives in which he tried to convince Julian to resign the title of Augustus and be satisfied with that of Caesar. By 361, Constantius saw no alternative but to face the usurper with force, and yet the threat of the Sassanids remained. Constantius had already spent part of early 361 unsuccessfully attempting to re-take the fortress of Ad Tigris. After a time he had withdrawn to Antioch to regroup and prepare for a confrontation with Shapur II.[ The campaigns of the previous year had inflicted heavy losses on the Sassanids, however, and they did not attempt another round of campaigns that year. This temporary respite in hostilities allowed Constantius to turn his full attention to facing Julian.

Death

Constantius immediately gathered his forces and set off west. However, by the time he reached Mopsuestia in Cilicia, it was clear that he was fatally ill and would not survive to face Julian. Apparently, realising his death was near, Constantius had himself baptised by Euzoius, the Semi-Arian bishop of Antioch, and then declared that Julian was his rightful successor.[80] Constantius II died of fever on 3 November 361.