The Siege of Belfort (3 November 1870 – 18 February 1871) was a 103-day military assault and blockade of the city of Belfort, France by Prussian forces during the Franco-Prussian War. The French garrison held out until the January 1871 armistice between France and the German Empire obligated French forces to abandon the stronghold in February 1871. 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.