The Belfort Gap is first recorded as playing a significant military role in 58 BC, when Julius Caesar marched his army through it to confront the Germanic chieftain Ariovistus, whom he defeated at the Battle of the Vosges (probably fought in the vicinity of modern Mulhouse). Over the following decades the whole area west of the Rhine was incorporated into the Roman Empire. The Western Roman Empire collapsed in the 5th century, and in the ensuing Migration Period the Germanic Alemanni settled in the area immediately east of the Belfort Gap, while the Burgundians passed through it and established the Kingdom of the Burgundians to the west. Both Burgundy and Alemannia were conquered by the Kingdom of Francia in the early 6th century, but the Belfort Gap remained the linguistic border between Germanic and Romance languages.