Battle of Mons Seleucus

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.

Background
Following his defeat at Mursa, Magnentius fled to Aquileia and summoned all his supporters to join him. Decentius, brother of Magnentius and newly-appointed Caesar, was engaged with an incursion of Alemanni, and could not afford to send any of his forces to Aquileia.

Constantius spent his time recruiting troops and retaking towns occupied by Magnentius. In the summer of 352 he moved into Italy, only to find that Magnentius had chosen not to defend the peninsula.

Battle
The armies of Constantius and Magnentius met at Mons Seleucus, in what is now La Bâtie-Montsaléon in Hautes-Alpes, south-eastern France. According to the historian Peter Crawford, it is not possible to fix the exact date of the battle from surviving sources. The rivals' prior movements are also unclear; Eutropius states that the usurper was "defeated in several battles", possibly leading up to the final battle at Mons Seleucus. Regardless, Constantius was again victorious and Magnentius killed himself on 10 August 353. Following his conclusive battle, Constantius wintered his troops at Arles.

Aftermath
Constantius, now undisputed Emperor of the Roman Empire, appointed Julian Caesar over the western half of the Empire in 355/6, and instigated a campaign to persecute those who had supported Magnentius. According to Ammianus Marcellinus, Constantius had become more "cruel, violent, and suspicious with age", and his notarii and bodyguards needed no pretext beyond mere suspicion to inflict punishment.