The Battle of Frigidus, or the Battle of the River Frigid, took place between September 5 and 6, 394, pitting the Roman Emperor Theodosius I´s army against the army of the western usurper, Eugenius. As a result of Eugenius and his Frankish magister militum, Arbogast´s, defeat, the Roman Empire was in the hands of a sole emperor for the last time in its history.
As a Christian, Theodosius was enraged by the pagan awakening that was taking place in the West under the reign of Eugenius. Furthermore, the death of Valentinian remained unsolved, and Eugenius had deposed all the civilian offices appointed by Theodosius when he ceded the western part of the empire to Valentinian II, leaving him with no control over the western Roman Empire.
When a Western embassy arrived at Constantinople to request Eugenius be recognized as the western augustus, Theodosius stayed apparently neutral. He received them with presents and vague promises. At this point it wasn´t clear that he had decided upon an offensive against Eugenius and Arbogast. Nevertheless, after appointing Honorius (his two-year-old son) as Western Roman Emperor in January 393, Theodosius decided to invade the West.
During the next year and a half, Theodosius formed his army for the invasion. The eastern armies were tremendously fragile ever since the death of emperor Valens, and the loss of most of his men in the Battle of Adrianopole. Now it was his task to impose discipline with the remaining Roman legions and reuniting forces in the reserve, as well as new recruits.
Victory was not assured and Theodosius, devoutly Christian, sought prophecies from a renowned hermit monk known as John of Egypt in Lycopolis, who foresaw a great (but costly) victory for Theodosius over Eugenius and Arbogast.
The eastern army encamped to the west of Constantinople by May 394. The legions were reinforced by auxiliary Barbaric troops, including around 20,000 Visigoths, and additional forces from Hispania and Syria. Theodosius commanded the forces in person, together with his generals Stilichos, Timasius and Bacurius, and the Visigothic King Alaric.
Their advance through Pannonia and the Julian Alps found no opposition, which made Theodosius and his men suspicious. Arbogast considered that the best strategy (based on his own experience while fighting Magnus Maximus in Gaul) was to have all his forces together to defend the Italian Peninsula, and therefore, left the Alpine way without surveillance. Arbogast’s´ forces were mainly Frankish and Gallic–Roman troops, as well as his own Goth auxiliary forces.
This is how Theodosius´ army managed to pass through the mountains without losses, and march to the Valley of River Frigidus, near to the Roman port of Aquilea. In the first days of September, Theodosius attacked the western army without previous examination of the battlecamp. He sent his Goth allies first, and the attack led to massive losses on both sides, including his general Bacurius.
Towards the end of the day, Eugenius kept a defensive strategy, while Arbogast sent detachments to close the mountain passage behind the Eastern troops. The next morning, Theodosius, encouraged by the news that Arbogast´s detachments had joined his troops, attacked again.
This time, a fierce tempest took place, with strong winds blowing in the faces of the Western army, blinding them with dust. Eugenius and Arbogast´s troops broke, and Theodosius obtained a decisive victory.