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.