Alsace–Lorraine, officially the Imperial Territory of Alsace–Lorraine, was a former territory of the German Empire, located in modern day France. It was established in 1871 by the German Empire after it had occupied the region during the Franco-Prussian War. The region was officially ceded to the German Empire in the Treaty of Frankfurt. French resentment about the loss of the territory was one of the contributing factors to World War I. Alsace–Lorraine was ceded to France in 1920 as part of the Treaty of Versailles following Germany's defeat in the war, although already annexed in 1918.