The battle between the armies of Roman Emperor Theodosius and usurper Eugenius on 5 and 6 September 394 in the upper Vipava Valley the only event of global historical significance in the entire ancient history of the area. Speaking of its great importance is the large number of mentions in ancient and medieval literature. While a single record sheds light on most of events in military and political history, or by two to three at the most, one comes across a description or at least a mention of the Battle of the Frigidus in most historians of late antiquity who describe the era of Emperor Theodosius. Their descriptions serve as basis for presentations of this event by Byzantine and Western Medieval writers of chronicles and church histories. Despite the large number of reports, a reconstruction of the event that would critically take into account all ancient records on the one hand, and the geographical and strategic characteristics of the battle site on the other, is an exceptionally complex task and also hypothetical in certain important segments. This is also because of the absence of any material finds that could provide a reliable source of the detailed course of the battle.
A detailed explanation for the curious ones:
Dr. Rajko Bratož has arrived to the following conclusions based on his studies of modern Latin sources, reports by Greek historians from the first half of the 5th century and their imitators up to the beginning of the 7th century, Latin summaries and translations from late antiquity, reports by Byzantine chroniclers and church historians from the 9th to 14th centuries and Western writers from the 8th to 14th centuries: The civil war battle between Theodosius and Eugenius started with a campaign by Theodosius. The emperor started it after extensive preparations in the first months of 394, which on the one hand concerned military, diplomatic and political, and family and dynastic affairs, and on the other hand primarily the religious, spiritual and ideological matters (visiting holy places, prayers and fasting, inquiring about the outcome of the future war with the hermit John of Lycopolis). After recruiting the units largely from the frontier of the eastern provinces, the emperor and the large army set out from Constantinople in the second half of May 394. In Thrace, he was joined by many "barbarians", especially Goths (more than twenty thousand men, according to Jordanes), as well as members of other peoples from the southern Danube basin and from the provinces north of the Black Sea, including Huns and Alans. Theodosius’s army was advancing along the main transport link in the Balkan and Danube provinces and reached the Barrier of the Julian Alps in just over two months (from the end of June to the beginning of September). The reports apparently refer to the events on the Hrušica Plateau (Ad Pirum), which were highlighted by the archaeological research from two decades ago. The Ad Pirum fortress apparently fell after a serious clash, which is witnessed by monetary and other finds. The fortress was most probably conquered by Theodosius's allied advance party, which consisted primarily of Goths.
Eugenius's army, which was commanded by Arbogast, who was of Frankish origin, consisted of western Roman units, from the army of Gaul with and with many Germanic mercenaries, Franks in particular. Its size is not known (only Socrates estimated it at "several tens of thousands men"). He left Milan with his army before the first of August 394. By the beginning of September, the army had deployed to defend the exit from the Julian Alps. Arbogast's plan was a major novelty in ancient warfare. The essence of the plan was the following: not defending the access to the defensive zone at any cost, but after the enemy arrives in the difficult transitional and specially defended territory, preventing it from leaving the defensive zone. Serving to this end was the encirclement manoeuvre of Arbition's army group, which sealed Theodosius's army in a difficult and hilly terrain between Logatec and Ajdovščina, where they would not able to get water or sufficient horse fodder in the forested karst area if they had to stay there for a longer period of time. The majority of Theodosius's army was late with combat arrangements and combat readiness because of the difficult routes, large size and extensive auxiliary staff. In front of the majority were barbarian federal units, which were the first to approach the enemy army. This was followed by the first day of the battle (5 September), to which sources pay very little attention. From the high pass, Theodosius saw in the plain in front of him the opposing army, consisting of infantry and cavalry, ready to fight. Believing that breaking through the barrier at the exit to the valley would be very difficult, he sent federal units to attack the enemy, perhaps precisely because he wanted to protect Roman units from excessive "blood tax"; this is said to have been paid by the "barbarians" (Arian Goths and others who, with the partial exception of the Caucasian peoples, were pagans). The attack by the federal units, which were dominated by Gothic cavalry and infantry (probably fighting alongside them were mounted archers, meaning Alans, Huns, Iberians or members of other eastern peoples), failed. Ten thousand Goths or half of the Gothic contingent is said to have fallen on the battlefield. Theodosius tried to help his federal allies in distress, but this was impossible due to the narrow paths and auxiliary staff. His army was late in preparing for battle, which the emperor noticed with a great deal of concern. At the critical moment of the utter defeat of the allied advance party, which could not be aided by the majority of the army, he dismounted and stepped in front of the column while shouting an adapted verse from the Psalms (115.2; "Ubi est Theodosii deus"). The result of the first day of the battle had a decisive influence on the events of the next day. It is said that after the failure of the federate allies, commanders advised Theodosius to withdraw the army and prepare a stronger force for next spring, with which he would win in any case. As this would mean an obvious defeat, Theodosius, convinced in the correctness of the prophecy of the hermit John of Lycopolis and thus of the victory of the Christian god, rejected the proposal. Theodosius's decision to continue the unfinished battle was reaffirmed by Arbition's army group switching to his side and by Flavianus's suicide. The motivation for part of the Eugenius's army defecting is unknown and could be only speculated on. Perhaps this was owing to the pro-Christian orientation of the commander himself, him being susceptible to bribery or the belief that Theodosius's side will win. The reports that Theodosius prayed and cried all night are strongly hagiographical. Another hagiographical feature of the story is also Theodosius's dream vision about heavenly horsemen. After the first, victorious day, Eugenius's army is said to have started to celebrate prematurely, also due to the recklessness of Eugenius himself, believing that after suffering such a severe defeat the day before, the opposing army was no longer capable of engaging in battle. After Theodosius's morning prayer, a mass attack by the majority of the Eastern army was mounted from the high rock above Vrhpolje at a time when Eugenius's army was not yet ready for battle. The attack started with Theodosius's sign of the cross shortly before hurricane-force winds picked up, pushing Theodosius's army towards the enemy, and making it very difficult for them to fight back. The missiles of Eugenius's army lost all their power and are said to have turned back by the hurricane-force winds, which tore the shields off the soldiers' hands, and the dust was forced into their eyes, obstructing their vision. The push through the enemy's ranks was led also on the second day by an allied commander, Bacurius the Iberian with (apparently eastern barbarian) units, who staged a true carnage of Eugenius's army. After the enemy ranks were broken through and their fortifications burned down, the pursuit and killing of enemy soldiers continued all the way to the Frigidus (Vipava), the flow of which is said to have been stopped by the piled up bodies. Eugenius was captured. When he was brought before Theodosius in restraints, he pleaded for mercy, but soldiers immediately beheaded him. His head is said to have been put on a stake and showed around the battlefield, and perhaps even in Italian cities after the battle. The remaining Eugenius's soldiers surrendered, pleaded Theodosius for forgiveness, and were immediately pardoned on the battlefield. The clash ended with the pursuit of the runaway commander Arbogast, who took refuge in the impassable terrain of the surrounding hills and committed suicide two days later by throwing himself at his own sword. While the number of casualties in the battle is unknown, it certainly by far exceeded twenty thousand soldiers. The news of the victory started spreading across the country, and it was officially announced in large cities, such as Alexandria.
The main question after the civil war ended was the attitude towards the defeated opponent. Theodosius was inclined to general amnesty for various reasons. He was convinced that he had won the battle with the help of god, so he himself had no right to punish the enemy. Bishop Ambrose of Milan, who had an exceptionally strong influence on the emperor, reaffirmed the ruler's belief in the divine nature of the victory.