A state (Land) of northern Germany, formerly the subject of a long-running dispute involving Prussia, Austria, and Denmark (the Schleswig–Holstein question). Both Schleswig and Holstein were originally duchies owing allegiance to the Danish crown. At the Congress of Vienna (1814) Holstein was incorporated into the Austrian-led German Confederation. In 1848 Denmark incorporated Schleswig, but the German-speaking population gained support from the German Parliament at Frankfurt and Prussian troops invaded Denmark. Britain, Russia, and France intervened to oblige Prussia to agree to an armistice, and under the London Protocol of 1852 Denmark retained its traditional rights in the duchies. However, Denmark in 1864 again incorporated Schleswig, provoking Prussian and Austrian troops to invade and defeat the Danish army. In 1866, after the Austro-Prussian War, Prussia annexed both duchies. Following World War I there were plebiscites and much of north Schleswig passed to Denmark as the province of South Jutland. Between the wars the existence of a German minority in the province created considerable tension. After World War II over three million refugees from East Germany crowded into Schleswig–Holstein and the area was reorganized to become a West German state.