Magnentius in the context of Roman civil war of 350–353

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.

Vetranio (died c. 356 AD) was briefly an imperial usurper and emperor in the Roman Empire in 350, during which time he controlled Illyricum between the rival emperors Magnus Magnentius and Constantius II, eventually capitulating to the latter.

The Great Conspiracy was a year-long state of war and disorder that occurred near the end of Roman rule in Britain. Fourth-century Roman historian Ammianus Marcellinus describes it as a barbarica conspiratio ('barbarian conspiracy') which took advantage of a depleted military force in the province; many soldiers had marched with Magnentius in his unsuccessful bid to become emperor. Few returned, and supply, pay, and discipline in the following years may have been deficient.

The precise chronology of events remains unclear. Ammianus, although being the author of the most comprehensive and widely-cited surviving sources on the Conspiracy, was living in Antioch during the time that it took place. His account is thus likely to be derived from other sources and exhibits inconsistencies with other known documentation.

Justina (Latin: Iustina; c. 340 – c. 388) was a Roman empress. She was initially the wife of the rebel emperor Magnentius and was then married to Valentinian I, with whom she had four children, including the emperor Valentinian II and the empress Galla.

Possibly a relative of the Constantinian dynasty, she was Valentinian's second wife after Marina Severa, and stepmother of the augustus Gratian and the mother-in-law of the augustus Theodosius I. Her young son Valentinian was made emperor shortly after her husband's death in November 375. According to Late Antique ecclesiastical history, Justina was an Arian Christian, and began to promote this christology after her husband died, bringing her into conflict with Ambrose, the Nicene Christian bishop of Mediolanum (Milan). In 387, fleeing from the invasion of the Italian Peninsula by the emperor Magnus Maximus, Justina took her children to the Balkans – including the child-emperor Valentinian II – and secured the intervention of the eastern emperor Theodosius in the civil war by marrying her daughter Galla to him at Thessalonica. Afterwards, Theodosius attacked and defeated Magnus Maximus, ending the civil war, during which time Justina herself died.

The Battle of Mons Seleucus was fought in 353 between the armies of the Roman emperor Constantius II and the usurper Magnentius. Constantius' forces were victorious. Support for Magnentius had been eroding since his defeat at the Battle of Mursa Major two years prior; after Mons Seleucus his cause collapsed and he killed himself.