Belfort is located in a gap between the mountainous southern Vosges and the Jura Massif, strategically positioned as the gateway between Alsace and central France.

Upon hearing of the approaching German army, Pierre Philippe Denfert-Rochereau, commander of Belfort, began constructing fortifications around the city, expanding those originally built by Vauban.

On 15 January 1871 Bourbaki attacked Werder along the Lisaine River; however after a three-day battle he was repelled and his army retreated into Switzerland.

Louis Adolphe Thiers, president of the Government of National Defense sent an urgent message to Denfert-Rochereau ordering him to surrender the fortress.

[2] In recognition of the French defense of Belfort, under the terms of the Treaty of Frankfurt, the city and its surrounding area were not handed over to Germany, unlike the remainder of Alsace.