Magnus Magnentius (c. 303 11 August 353) was a Roman general and usurper against Constantius II from 350 to 353. Of Germanic descent, Magnentius served with distinction in Gaul under the Western emperor Constans. On 18 January 350 Magnentius was acclaimed augustus. Magnentius quickly killed the unpopular emperor and gained control over most of the Western Empire. The Eastern emperor Constantius II, the brother of Constans, refused to acknowledge Magnentius' legitimacy and led a successful campaign against Magnentius in the Roman Civil war of 350-353. Ultimately, Magnentius' forces were scattered after the Battle of Mons Seleucus, and he committed suicide on 11 August 353.Much of Magnentius' short reign was concerned with asserting his legitimacy. Unlike Constans, Magnentius was unrelated to Constantine the Great, and so had no dynastic claim to the emperorship. Magnentius instead sought popular support by modeling himself as a liberator who had freed the Western Empire from the tyranny of Constans. Though attempted public and religious reform, his acts were quickly repealed by Constantius after his death. In light of the political instability of his reign, most modern and ancient historians consider him a usurper rather than an emperor.

Perhaps the most important consequence of Magnentius' revolt was the severe depletion of the Empire's military forces in civil war: The Battle of Mursa left so many Roman soldiers dead that, according to Zosimus, Constantius feared that Rome would no longer be able to effectively hold off barbarian invasion. Following his death, Constantius II became the sole emperor of the Roman Empire.

Flavius Julius Constantius (Greek: Κωνστάντιος; 7 August 317 – 3 November 361), known as Constantius II, 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 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 heresy, banned pagan sacrifices, and issued laws 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 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' 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.