AD 353

Battle of Mons Seleucus

Constantius II defeats Magnentius at Mons Seleucus

A late-antique source says Magnentius escaped the battlefield alone before making for Lyon.

Two Roman armies in the Hautes-Alpes

In 353 the armies of Constantius II and of Magnentius met at Mons Seleucus, and Constantius’s victory ended Magnentius’s ability to sustain his claim to the imperial throne. Both armies belonged to the same empire; this was a civil war, not a defence against a foreign invasion.

Magnentius had already been beaten expensively at Mursa in 351 and had lost Italy in 352. Constantius carried the war into Gaul, and Mons Seleucus was where Magnentius made his last effective military resistance.

The ancient settlement is identified archaeologically with La Bâtie-Montsaléon, in France’s Hautes-Alpes.

Alone to Lugdunum

A late-antique account says Magnentius escaped alone from the encounter at Mons Seleucus and fled to Lugdunum, modern Lyon.

Eutropius records that Magnentius, defeated in multiple engagements, died by suicide at Lugdunum, and that his Caesar Decentius also killed himself, at Sens. The deaths are commonly dated to 10 and 18 August 353.

Those dates are the usual modern choice rather than settled fact: ancient chronological traditions give 10, 11 and 15 August for Magnentius, and a minority modern reconstruction has treated both men as executed instead.

A day the sources never fix

The year 353 is secure, but the surviving evidence does not establish the battle’s exact day. Modern reconstructions variously place it in July or in August.

The battle and the suicide were separate events in different places, and should not be run together into a single day. Magnentius died at Lugdunum, not on the field at Mons Seleucus.

Constantius consequently became the empire’s sole Augustus, although unrest and external attacks in Gaul continued.

Sources

Researched 23 Aug 2026 8 sources not yet audited

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What the sources leave uncertain

The year 353 is secure, but the surviving evidence does not establish the battle’s exact day; modern reconstructions variously place it in July or August.
Magnentius’s death is commonly dated to 10 August, but ancient chronological traditions differ, including 10, 11 and 15 August.
The suicide narrative is supported by Eutropius and other ancient accounts, although a minority modern reconstruction has treated Magnentius and Decentius as executed.
The battle and Magnentius’s suicide were separate events in different places and should not automatically be presented as occurring on the same day.