19 July 1870, Napoleon III declared war on Prussia. The French armies were quickly defeated. In Alsace the french army lost a few battles in  Wissembourg and Frœschwiller and later in Strasbourg. In Moselle at Gravelotte, Mars la Tour and Saint-Privat…

The Maréchal Bazaine was surrounded in Metz. This has led MacMahon to make a strategic mistake and both Bazaine and MacMahon were trapped in Sedan. 2 September 1870 after the defeat at Sedan, the Second Empire was overthrown. A few brave and resistant strongholds should be noted: Thionville, Bitche, Phalsbourg, Neuf-Brisach, and Belfort.

The establishment of the Republic did not prevent France from surrendering on January 28, 1871. The german empire is proclaimed the 26 february 1871  in Versailles. 

The 10th of may  the Treaty of Frankfurt  made the territorial annexation official: Alsace-except for Belfort—but enlarged with the Upper Bruche Valley (Schirmeck – Saâles) and Moselle (without Briey, but with Sarrebourg and Château-Salins) were handed over on Germany. This was the first annexation and it was legal since it was voted  by the French National Assembly gathered in Bordeaux, despite the protests of the deputies from Alsace and Moselle.

In June 1871, the three French departments of Bas-Rhin, Haut-Rhin and Moselle became the Reichland Elsaß Lothringen. According to the Treaty of Frankfurt, 160,000 people choose to remain french, and 50,000 of them left the german territory to go to France. At the end of the legal emigration period, many young people attempted to avoid the military service in the German Army, which was made mandatory since 1872. German government workers and soldiers were send in Alsace-Moselle to replace those who decided to left.

Alsace and Moselle were to develop within a new government, the Empire of Wilhelm II and the Bismarckian regime, which invested heavily in the new annexed territories.
In the 1890’s, Alsace and Moselle experienced a phase of appeasement. THe economic prosperity, the introduction of advanced social legislation and the constitution of 1911 were the main factors of this appeasement.