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 invasions.

Born in Samarobriva (Amiens), Gaul, Magnentius was purportedly the son of a British father and a Frankish mother,[11][12][13] though some scholars suspect that his supposed origins were exaggerated or invented as a result of Constantinian propaganda.

[17][3] 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 usurpation.

[18][11][17] 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).

[21][23] Magnentius quickly attracted the loyalty of the provinces of Britannia, Gaul, and Hispania, and immediately moved to control Italy.

Lacking a connection to the Constantinian dynasty, Magnentius married Justina, possibly a great-granddaughter of Constantine (and future wife of Valentinian).

[11] Magnentius also appealed to the controversial bishop Athanasius—who in the end chose to side with Constantius—and repealed some of Constans' restrictions on pagan sacrifice in Italy.

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.

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.

[35] Contemporary commentators like Eutropius lament the waste of Roman soldiers, noting that enough men for an entire foreign invasion were lost.

[34] Magnentius blocked the passages West, and Constantius engaged in skirmishes with the barbarians to the Northeast for the rest of the year.

Constantius crossed the Alps in 353,[8] 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).

[46] 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 fourth century that he could use it freely.