1784 boundary marker on the Chemin de Bussurel

The Territory of Belfort was born in 1871 from the Treaty of Frankfurt which put an end to the war of 1870-1871. It is then called "subsisting district of Haut-Rhin". Alsace and Lorraine had the particularity of being two bilingual regions and their dismantling was organized according to linguistic criteria. The German Empire thus gained by this treaty the greater part of Alsace and a quarter of Lorraine. The extreme south-west of the Haut-Rhin, around Belfort, was left to France and thus separated from the rest of Alsace. The main reason is that the population around Belfort was predominantly French-speaking (while the rest of Alsace was predominantly German-speaking, with the main Alsatian dialect being part of the Germanic languages). However, this was not an absolute criterion because there were French-speakers and German-speakers both in Belfort and in the rest of Alsace, only their proportion differed.