But Constantius, though he granted fair terms of pardon and reconciliation to all who abandoned the standard of rebellion, avowed his inflexible resolution to inflict a just punishment on the crimes of an assassin, whom he prepared to overwhelm on every side by the effort of his victorious arms.
An Imperial fleet acquired the easy possession of Africa and Spain, confirmed the wavering faith of the Moorish nations, and landed a considerable force, which passed the Pyrenees, and advanced towards Lyons, the last and fatal station of Magnentius.

The temper of the tyrant, which was never inclined to clemency, was urged by distress to exercise every act of oppression which could extort an immediate supply from the cities of Gaul.

Their patience was at length exhausted; and Treves, the seat of Praetorian government, gave the signal of revolt, by shutting her gates against Decentius, who had been raised by his brother to the rank either of Caesar or of Augustus.

From Treves, Decentius was obliged to retire to Sens, where he was soon surrounded by an army of Germans, whom the pernicious arts of Constantius had introduced into the civil dissensions of Rome.

In the mean time, the Imperial troops forced the passages of the Cottian Alps, and in the bloody combat of Mount Seleucus irrevocably fixed the title of rebels on the party of Magnentius.