350–353 Late period The Battle of Mons Seleucus was fought in 353 between the armies of the Roman emperor Constantius II and the usurper Magnentius.

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.

Decentius, brother of Magnentius and newly-appointed Caesar,[1] was engaged with an incursion of Alemanni, and could not afford to send any of his forces to Aquileia.

[3] 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 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.