|Roman emperor |
(unrecognized in the East)
|Reign||18 January 350 –|
11 August 353
|Died||11 August 353 (aged c. 50)|
Lugdunum (Lyon), Gaul, Roman Empire
|Issue||Unnamed daughter[note 1]|
|Religion||Disputed; See below|
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.[note 3]
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 Though attempted public and religious reform, his acts were quickly repealed by Constantius after his death.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.
Born in Samarobriva (Amiens), Gaul, Magnentius was the son of a British father and a Frankish mother. He served in the protectores, a prestigious bodyguard corps attached to the imperial retinue, and was promoted to comes of the Herculians and Jovians, the Imperial guard units. Magnentius seems to have been effective in his post and to have been well-liked by other important court officials, notably Marcellinus, the comes rerum privatarum, who supported his userpation.
Court officials eventually hatched a plot to overthrow Constans and place Magnentius in power. By 350, Constans was likely deeply unpopular; ancient sources deride him as licentious, incompetent, and homosexual. There is confusion as to the details, but in Augustulum (Autun) on 18 January 350 Magnentius announced his bid for power by attending a dinner party, stepping out for a moment, and re-entering the room in a purple toga (the traditional dress of an emperor). The party-goers, either all or in part in on the plot, hailed him Augustus.
In the days following his acclimation, Magnentius bribed the city guard to secure their loyalty and barred residents of Augustulum from leaving the city to prevent the spread of the news. Anxious to eliminate Constans—who was then hunting near the Pyrenees away from his forces—Magnentius sent a contingent to run him down, and Constans was killed at Helena (near Perpignan). Zosimus claims this force was headed by Gaiso, and that his consulship in 351 was a reward for this act. Magnentius may also have executed a purge of Constans' supporters by forging letters from Constans purportedly summoning them to court; when these supporters arrived, they were killed on the spot.
Magnentius quickly attracted the loyalty of the provinces of Britannia, Gaul, and Hispania, and immediately moved to control Italy. He did so within a month of his ascension. and appointed Fabius Titianus to the position of praefectus urbis. He seems to have secured Africa quickly as well, thus inheriting all of Constans' former territory except Illyria (which would shortly come under the control of Vetranio).
Magnentius' early reign is marked by a series of bids for political legitimacy. Lacking a connection to the Constantinian dynasty, Magnentius married Justina, great-grandaughter of Constantine (and future wife of Valentinian). Magnentius aimed to position himself as a liberator in the public eye, and minted coins with such inscriptions as 'liberator of the Roman world.' Further, Magnentius seems to have initially pursued a policy of diplomacy with Constantius, hoping to have the emperor recognize him officially: for the first 6 months of his reign, Magnentius spoke of Constantius as his senior and minted coins bearing his image. Magnentius also appealed to the controversial bishop Athanasius—who in the end chose to side with Constantius—and repealed some of Constans' restrictions on pagan sacrifice in Italy. This latter decision, aimed at winning the support of wealthy Italian pagans, proved to be a rather crude political maneuver, and earned Magnentius a reputation as a pagan himself.
Despite his efforts, however, Magnentius remained politically vulnerable. Constantius reasoned that legitimizing the man who murdered his brother would set an unacceptable precedent, and so prepared to march West. In Italy, Magnentius' hold over Rome proved tenuous, and Julius Nepotianus, Constantine's nephew, took and held the city for 27 days. Magnentius dispatched Marcellinus (now promoted to magister officiorum) to retake the city. In Illyria, Vetranio was acclaimed emperor with the support of Constantus' sister Constantina on 1 March 350. Though he publicly allied with Magnentius, many modern scholars contend that Vetranio was working for Constantius from the beginning, preventing Magnentius from threatening Constantinople until Constantius could march West.
Main article: Roman civil war of 350–353
By the summer of 350 it had become clear that war was inevitable between Constantius and Magnentius. Magnentius had already moved east over the spring, and appointed his brother Decentius as caesar to control Gaul during his absence. In the East, Constantius was tied down for most of 350 in his War against the Persians. However, in the autumn of 350 Constantius successfully lifted the Persian siege of Nisibis and forged a nominal piece with the Sasanids. Appointing Constantius Gallus caesar and entrusting command of the Eastern legions to Lucillianus, Constantius moved West. On 25 December 350 he accepted Vetriano's resignation in Illyria. Modern scholars see the event as planned, but ancient sources describe Constantius as conquering by rhetorical ability alone.
Constantius and Magnentius wintered far apart, and, in the early months of 351, both parties flirted with the idea of peace. Constantius offered Magnentius control of Gaul, which Magnentius rejected. Magnentius' force may have been slowly deserting him, for he seems to have pushed for a battle: either the night before the battle or long before (accounts conflict) the important commander Silvanus defected to Constantius. After failing to take the city of Mursa, Magnentius' force was approached by Constantius', and the Battle of Mursa Major ensued. Constantius won a Pyrrhic victory in one of the largest battles ever fought among Romans: low estimates suggest there were over 50,000 deaths. The sight of the field is reported to have reduced Constantius to tears, and to have made him fear that Rome would be unable to resist Barbarian invasion. Contemporary commentators like Eutropius lament the loss, noting that enough men for an entire foreign invasion were lost.
Magnentius retreated across the Alps, perhaps after being prevented from entering Italy by the erection of garrisons loyal to Constantius. Magnentius blocked the passages West, and Constantius engaged in skirmishes with the barbarians to the Northeast for the rest of the year.
352 is marked by Magnentius' failed attempts to broker a peace with Constantius: Magnentius sent an embassy to Constantius who, worried about espionage, prevented them reaching him. Magnentius sent an assembly of bishops to Constantius with the message that he was willing to step down and take a soldier's posting under Constantius. Constantius rejected this offer as well. Legions along the Rhine began defecting en masse, access to northern Gaul and Spain was blocked, and Magnentius was unable to effectively prevent defection even in his camp.
At some time in the year 353, Magnentius may have attempted to assassinate Constantius Gallus, Constantus' Caesar in the East, in a bid to open the Eastern empire to attack. The incident is mentioned only in the account of Zonaras, though some scholars see a passage in Ammianus Marcellinus as corroborating it. However, some modern scholars have claim that Zonaras' account contains chronological problems, and thus the historicity of this event is uncertain. In any case, the plot was foiled.
Constantius crossed the Alps in 353, and Magnentius made a final stand in July of the same year at the Battle of Mons Seleucus, after which he fled to Lugdunum (Lyon). There, some sources report that his home was surrounded by the forces of Constantius and that he lashed out against his brother Desiderius[note 4]. However, most sources agree that, on 10 or 11 August 353, he committed suicide by falling on his sword. His brother Decentius hanged himself when he received the news.
Following the suppression of Magnentius' rebellion, Constantius began to root out his followers. The most notorious agent he employed in this search was the primicerius notariorum Paulus Catena ("Paul the Chain").
Magnentius' religious beliefs remain a point of contention among intellectuals. Many scholars argue that Magnentius was, at least nominally, a Christian. However, some sources hint at a pagan restoration during his short reign. Epigraphic evidence suggests that he allowed pagans to perform sacrifices at night, a practice that had been banned by Constans. Philostorgius describes Magnentius as a demon worshiper, and Zonaras recounts a story of him using black magic.  Numismatically, Magnentius' coinage is neither distinctively pagan nor Christian, with the exception of a series of coins bearing a Christogram. This symbol has come to be associated solely with Christianity, but this may not reflect attitudes at the time: MacMullen argues that Magnentius was a Pagan, and that the Chi-Rho had been so de-Christianized by the mid 4th century that he could use it freely.