The Battle of the Frigidus, also called the Battle of the Frigid River, was fought on 5 and 6 September 394 between the armies of the Roman emperor Theodosius the Great and the rebel augustus Eugenius, in the eastern border of Roman Italy. Theodosius won the battle and defeated the usurpation of Eugenius and Arbogast, restoring unity to the Roman Empire. The battlefield, in the Claustra Alpium Iuliarum near the Julian Alps through which Theodosius's army had passed, was probably in the Vipava Valley – with the Frigidus River being the modern Vipava – or possibly in the valley of the Isonzo.

Valentinian II was a Roman emperor in the western part of the Roman empire between AD 375 and 392. He was at first junior co-ruler of his half-brother, then was sidelined by a usurper, and finally became sole ruler after 388, albeit with limited de facto powers.