Editing Constantius II (section)

===War against Magnentius===
{{main|Roman civil war of 350–353}}
Constantius was determined to march west to fight the usurper.{{sfn|Errington|2006|p=16}} However, feeling that the east still required some sort of imperial presence, he elevated his cousin [[Constantius Gallus]] to ''caesar'' of the eastern provinces.{{sfn|Barnes|1993|p=105}}{{sfn|Potter|2004|p=472}} As an extra measure to ensure the loyalty of his cousin, he married the elder of his two sisters, [[Constantina]], to him.{{sfn|Hunt|1998|p=17}}

Before facing Magnentius, Constantius first came to terms with [[Vetranio]], a loyal general in [[Illyricum (Roman province)|Illyricum]] who had recently been acclaimed emperor by his soldiers.{{sfn|Barnes|1993|p=101}} 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''.{{sfn|Potter|2004|p=472}} However, when Constantius arrived, Vetranio willingly resigned his position and accepted Constantius's offer of a comfortable retirement in [[Bithynia]].{{sfn|Hunt|1998|pp=16–17}}

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.{{sfn|Potter|2004|p=473}}{{sfn|Hunt|1998|p=20}} 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]].{{sfn|Potter|2004|p=474}} 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 [[Roman Gaul|Gaul]].{{sfn|Potter|2004|p=474}}{{sfn|Barnes|1993|p=106}}

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.{{sfn|Potter|2004|p=474}} Magnentius, realizing the futility of continuing his position, committed suicide on 10 August 353.{{sfn|Hunt|1998|p=22}}{{sfn|Drinkwater|2007|p=201}}{{sfn|Barnes|1993|p=106}}