Treaty of Frankfurt (1871)

Concluded after the armistice of 28 January and the preliminary peace of 26 February, the treaty fixed the new frontier. France ceded the Alsatian departments of Bas-Rhin and Haut-Rhin except Belfort and its territory, and in Lorraine most of the Moselle department, a third of Meurthe including Château-Salins and Sarrebourg, and the cantons of Saales and Schirmeck in the Vosges: 1,447,000 hectares and 20 per cent of its mining and steel potential went with them. Residents of the annexed region had until 1 October 1872 to keep French nationality and emigrate or to remain and become German citizens. The treaty set a framework for the staged withdrawal of German troops, regulated the payment of the five-billion-franc indemnity, required one billion in 1871 before any withdrawal and military occupation of parts of France until the sum was paid, recognised Wilhelm I as German Emperor, and dealt with navigable waterways connected with Alsace-Lorraine, trade between the two countries and the return of prisoners of war. The trade treaty of 1862 was not renewed, and France granted Germany most-favoured-nation treatment for trade and navigation. France paid faster than agreed and the German troops withdrew in September 1873.

Several factors shaped the boundary. The German military regarded Alsace up to the Vosges and the route between Thionville and Metz as a requirement for the protection of Germany in any future war with France. Without a westward shift, the new empire's frontier with France would have fallen largely to Baden and Bavaria, which would have required stationing imperial forces in those states and compromised the autonomy the southern states had kept in the unification treaty. The new border largely followed the linguistic one, and the fact that most of the population spoke German dialects and had belonged to the Holy Roman Empire two centuries earlier served as a nationalist justification, but the conquest of French-speaking Metz sparked outrage in France and became one of the main arguments of revanchism. Iron ore and coal do not appear to have played a role in the annexation. France respected the treaty in its entirety until 1914. The treaty polarised French policy towards Germany for forty years, and the recovery of the "lost provinces" became one of the most powerful motives for France's involvement in the First World War. In 1918 Wilson advocated the transfer of Alsace-Lorraine to France as the eighth of his Fourteen Points without reference to the wishes of its population, and it was returned to France under the Treaty of Versailles of 1919.