The Belfort Gap (French: trouée de Belfort) is a plateau located between the northern end of the Jura Mountains and the southernmost part of the Vosges mountains. Its altitude varies between 345 meters at its lowest and a little more than 400 meters in the area of the watershed between the catchment areas of the Rhine and that of the Rhône. This watershed divide was used in 1871 to determine the new border between the German Empire and France. A series of fortifications was set up to ensure defense of the Belfort Gap. This roughly 40-kilometer wide terrain corridor connects the département of Haut-Rhin, south of the région Alsace and the département Territoire-de-Belfort, north of the région Franche-Comté.[1] Lines of communication that traverse the Belfort Gap include national highway RN 83 and autoroute A36, the Paris-Basel railway and the planned high-speed railway "Rhine-Rhone", as well as the Rhône-Rhine Canal.
Two important routes crossed the area during the Roman era, which allowed Caesar to quickly move troops which were used to defeat Ariovistus and force his German tribes (primarily Suebi) to retreat across the Rhine. The town of Belfort, situated in the center of the corridor, has played the role of a bolt against invasions over the centuries whether originating from the west or the east and remained a communications center that is impossible to circumvent. The most recent military advance through the Belfort Gap was that of the French I Corps in November 1944.[2]