Theodosius first meets the forces of the usurper Eugenius, commanded by Arbogast, at the Frigidus River (modern Vipava River) on the eastern border of Italy, between Aquileia and Emona.

Theodosius' advance guard, composed almost entirely of Visigoth federates under Alaric on September 5, 394, suffers heavy losses during an attempted breakthrough and is only saved by nightfall.

The emperor ventures to attack the following day, however, and is victorious, due chiefly to the brilliant tactics of Stilicho.

The Battle of the Frigidus is part of a trend towards using increasing percentages of barbarian troops, especially in the west, where it will led to the weakening of the empire itself.

Eugenius had represented the last opportunity for the Pagans and the senatorial class to oppose the Christianization of the Empire.

Arbogast commits suicide immediately after the defeat, as does Nicomachus Flavianus, while Eugenius is held for execution as a criminal, his head afterward being displayed in Theodosius' camp.

Theodosius following the deaths of these three shows himself lenient and strives to achieve the settlement between opposing forces that is necessary to strengthen imperial unity.

He promotes Stilicho to generalissimo.