Battle of Frigidus
The Battle of the Frigidus, also called the Battle of the Frigid River, was fought between September 5-6 394, between the army of the Eastern Emperor Theodosius I and the army of Western Roman ruler Eugenius.
The defeat of Eugenius and his commander, the Frank magister militum Arbogast, put the whole empire back in the hands of a single emperor for the last time in Roman history. Most significantly, the battle was the last attempt to contest the Christianization of the empire, its outcome decided the fate of Christianity in the western Empire.
The mainly Pagan Roman Senate had been arguing with the Christian emperors in Constantinople and Milan for over two generations, since Constantine I had recognized Christian faith and Theodosius had made it the official religion of the State. The senators wrote letters and argued for a return to Paganism, often stressing the protection and good fortune the old Roman gods had bestowed Rome since her beginnings as a small city-state. For their part, the Christian emperors emphasized the primacy of Christianity, although not all did so to the same extent. This clash between the Roman world's two main religions was for the most part merely an academic debate, without threatenings of an armed uprising.
On May 15 392, however, the Western Emperor Valentinian II was found hanged at his residence in Vienne, Gaul. Valentinian, who for a time showed some favoritism towards the Arians, had continued the imperial policy of suppressing Pagan interests over those of the Christians. Not surprisingly, this policy had resulting in increasing tensions between the emperor and the Pagan senators.
When the Eastern Emperor Theodosius heard the news of Valentinian's death, Arbogast, who was the magister militum and de facto ruler of Western Empire, informed him that the young emperor had committed suicide.
Tensions between the two halves of the empire were heightened further that summer. Arbogast made several attempts to contact Theodosius, but apparently none got further than the ears of the Eastern praetorian prefect Rufinus. The responses that Arbogast received from Rufinus were unhelpful. Theodosius himself was slowly coming around to the belief Valentinian had been murdered, in no small part because his wife Galla was convinced her brother's death was caused by treachery.
For his part, Arbogast had few friends in the Eastern court. His chief ally was his uncle Ricomer, but he was suffering from a mortal illness. As it appeared increasingly likely that whatever course Theodosius decided upon would be hostile towards Arbogast, the Frank decided to make the first move.
On August 22 of that year, Arbogast elevated Flavius Eugenius, the Western imperial court's magister scrinii, to the purple. Eugenius was a well-respected scholar of rhetorics, and a better claimant to the purple than Arbogast himself. His ascension was backed by the Praetorian prefect of Italy, Nicomachus Flavianus the Elder, and also by many of the Pagan members of the Roman Senate. However, some senators, notably Symmachus, were uneasy with this action.
After his elevation to emperor, Eugenius appointed several important Pagan senators to key positions in the Western government. He also supported a movement to advance the Pagan cause by granting it official recognition and by restoring important Pagan shrines such as the Altar of Victory and the temple of Venus and Rome. These actions earned Eugenius withering criticism from Ambrose and did little to endear him to Theodosius.
As Christian, Theodosius was distressed by the Pagan revival that was occurring under Eugenius's reign. In addition there was the issue of Valentinian's death, which had never been resolved to his satisfaction. Furthermore, Eugenius had removed all the high civil officer left by Theodosius when he had given the Western half of the empire to Valentinian, so that Theodosius had lost his control on the Western Roman Empire.
When a party of Western ambassadors arrived in Constantinople to request that Eugenius be acknowledged as the Western Augustus, Theodosius was noncommittal, even if he received them with presents and vague promises. Whether he had already decided on an offensive against Eugenius and Arbogast at this point is unclear. In the end, however, after declaring his two-years-old son Honorius as the western Augustus in January of 393, Theodosius finally resolved to invade the West.
Over the following year and a half Theodosius marshalled his forces together for the invasion.
The Eastern armies had atrophied since the death of the emperor Valens and most of his men at Adrianople and it fell to the generals Flavius Stilicho and Timasius both to restore discipline to the Eastern legions and to bring them back up to strength through recruitment and conscription.
At the same time another of Theodosius's advisers, the eunuch Eutropius, was sent out from Constantinople to seek the advice and wisdom of an aged Christian monk in the Egyptian town of Lycopolis. According to the accounts of the meeting given by Claudian and Sozomen, the old monk prophesized that Theodosius would achieve a costly but decisive victory over Eugenius and Arbogast.
The Eastern army set out towards the west from Constantinople in May of 394. The regalvanized legions were bolstered by numerous barbarian auxiliaries including over 20,000 Visigoth federates and additional forces from Spain and Syria. Theodosius himself led the army, and among his commanders were his own generals Stilicho and Timasius, the Visigoth chieftain Alaric, and an Iberian named Bacurius.
Their advance through Pannonia of the Julian Alps was unopposed, and Theodosius and his officers must have had suspicions about what lay ahead when they discovered that the eastern ends of the mountain passes were undefended. Arbogast had, based on his experiences fighting against the usurper Magnus Maximus in Gaul, decided that the best strategy was to keep his forces united to defend Italy itself, and to that end he went so far as to leave the Alpine passes unguarded. Arbogast's forces consisted mainly of his fellow Franks and Gallo-Romans, plus his own Gothic auxiliaries.
Thanks to Arbogast's strategy of maintaining a single, relatively cohesive force, the Theodosian army passed unhindered through the Alps and descended towards the valley of the Frigidus River above Aquileia. It was in this narrow, mountainous region that they came upon the Western army's encampment in a pass near present-day Vipava, Slovenia, in the first days of September.
Theodosius attacked almost immediately, having undertaken little to no prior reconnaissance of the field of battle. He committed his Gothic allies into action first, perhaps hoping to thin their ranks through attrition and lessen their potential threat to the Empire. The Eastern army's headlong attack resulted in heavy casualties but little gain, and the Spanish general Bacurius was among the dead.
Day's end saw Eugenius celebrating his troops' successful defense of their position while Arbogast sent out detachments to close off the mountain passes behind Theodosius's forces.
After a sleepless night, Theodosius was cheered by the news that the men Arbogast had sent to bottle him up in the valley intended to desert to his side. Buoyed by this favorable development, Theodosius's men attacked once again. This time nature was on their side as a fierce tempest — apparently a regular occurrence in the region — blew along the valley from the east. The high winds blew clouds of dust into the faces of the Western troops. Buffeted by the winds, Arbogast's lines broke and Theodosius gained the decisive victory that the Egyptian monk had prophesied.
In the aftermath, Eugenius was captured and brought before the emperor. His pleas for mercy went unanswered and he was beheaded. Arbogast escaped the defeat and fled into the mountains, but after a few days' wandering, he concluded escape was impossible and committed suicide.
It had been a costly but total victory for Theodosius, and a total loss for the Pagans. The western provinces quickly submitted to Theodosius, who became the last emperor of a united empire.
Most significantly, the battle was the last attempt to contest the Christianization of the empire, its outcome decided the fate of Christianity in the western Empire. The battle is on par with that of the Battle of the Milvian Bridge in importance, for it was seen not only as a victory in a civil war, but a vindication of the Christian God and the triumph of Christianity - within a generation the elite Pagan families of Rome would give up any serious resistance to Christianity and re-invent themselves as the papal families of Late Antiquity.
Unfortunately, the battle also accelerated the collapse of the Roman army in the west. The legions were already losing their effectiveness due to reorganizations and a decline in the quality of their training and discipline, and the losses at the Battle of the Frigidus weakened the western legions — whose task in defending the empire from the barbarian invaders was much harder than the eastern ones — still further. This downturn in the capabilities of the Roman soldiers meant an increasing reliance by the Western Empire on barbarian mercenaries employed as federates, who often proved too unreliable, or worse.
 Kenneth W. Harl (2004). Rome and the Barbarians. The Teaching Company. ISBN 1-56585-903-0