The Roman civil war of 394 AD was fought between the Roman emperor Theodosius I and the Frankish general Arbogast and his puppet usurper, Eugenius. Because Eugenius, though nominally Christian, had pagan sympathies, the war assumed religious overtones, with Christianity pitted against the last attempt at a pagan revival.

Arbogast was a powerful Frankish general (magister militum) employed in the Roman army, who had managed to pacify the barbarians in Gaul and along the Rhine frontier on behalf of the Empire in 389 AD. In a naked grab for power, he turned against the Western Roman emperor, Valentinian II, and instigated his murder, though he declared Valentinian's death to be a suicide. He then proclaimed his supporter, Eugenius, a former rhetoric teacher, as emperor in 392 AD. Eugenius then spent months attempting to gain legitimacy by winning over Theodosius.

Refusing to recognise the usurpers, in January 393 the Eastern emperor Theodosius I elevated his son Honorius as the Augustus of the West, and thus his co-emperor. He marched with a large army to avenge Valentinian's death, travelling from Constantinople to northeastern Italy. Here, Theodosius confronted Arbogast, Eugenius and their forces at the Battle of the Frigidus River, near Aquileia. The first day of battle (September 5, 394 AD) ended in near defeat for Theodosius, and he was saved only by the coming of night. During that long night he prayed to his Christian God, where it was said that Theodosius was visited by two "heavenly riders all in white"[1] who gave him courage. The next day, he reorganised his troops and as the battle began again and Theodosius' forces were aided by a natural phenomenon known as the Bora,[2] which produces cyclonic winds. The Bora blew directly against the forces of Eugenius and disrupted the line, allowing the tactical brilliance and masterful generalship of Theodosius' commander, Stilicho, to win the day.

Eugenius was killed in his camp and Arbogast committed suicide, and with their deaths, the pagan counterrevolution in Rome also died. Theodosius once again reunited the empire under the authority of one emperor, but it would not last. By 395 AD he was dead, and his two sons re-divided the empire, permanently this time.