The German states proclaimed their union as the German Empire under the Prussian king, Wilhelm I, uniting Germany as a nation-state. The Treaty of Frankfurt of 10 May 1871 gave Germany most of Alsace and some parts of Lorraine, which became the Imperial territory of Alsace-Lorraine (Reichsland Elsaß-Lothringen).The German conquest of France and the unification of Germany upset the European balance of power, that had existed since the Congress of Vienna in 1815 and Otto von Bismarck maintained great authority in international affairs for two decades. French determination to regain Alsace-Lorraine and fear of another Franco-German war, along with British apprehension about the balance of power, became factors in the causes of World War I.