5 – 6 September – Later Roman Empire
The Battle of the Frigidus, also called the Battle of the Frigid River, was fought between 5–6 September 394, between the army of the Eastern Emperor Theodosius I and the army of Western Roman ruler Eugenius.
Because the Western Emperor Eugenius (though nominally Christian) had pagan sympathies, the war assumed religious overtones, with Christianity pitted against the last attempt at a pagan revival. The battle was the last serious attempt to contest the Christianization of the empire; its outcome decided the outcome of Christianity in the western Empire, and the final decline of Greco-Roman polytheism in favour of Christianity over the following century.
The defeat of Eugenius and his commander, the Frankish magister militum Arbogast, put the whole empire back in the hands of a single emperor for the last time until the final collapse of the Western Roman Empire (if one discounts the purely nominal claim of Zeno in 480). Theodosius passed the rule of the Western Empire to his younger son Honorius in the following year (with general Stilicho as regent while Honorius was under age).