On August 3, 1914, Germany declared war on France. Since July 31, the Reichsland was already under  a state of war: basics freedoms were suppressed, public meetings forbidden, press censored. The next day, the mobilization order was posted in all the cities. The military dictatorship was total, rationing was drastic, economic sequestration was numerous, repression was heavy and expulsions were incessant.

Moselle (battles of Sarrebourg and Morhange, August 18-20, 1914) and Alsace experienced terrible fighting (Linge, Hartmannswillerkopf in 1915). In the combat zones, there was a lot of misunderstanding : Alsatian and Moselle civilians were suspected by the belligerents: “Frenchies” for some, “Prussians” for others. Some, like Doctor Schweitzer, were interned in French camps.

During the Great War, 380,000 men from Alsace-Moselle were drafted into the German army. To avoid desertions, most of them were sent to the Russian front. 18,000 men from Alsace-Moselle joined the French Army.

In November 1918, the French troops entered Alsace and Moselle and were welcomed with enthusiasm. The Treaty of Versailles of June 28, 1919 reintegrated Alsace and Moselle into French sovereignty.