Southern Schleswig (German: Südschleswig or Landesteil Schleswig, Danish: Sydslesvig) is the southern half of the former Duchy of Schleswig in Germany on the Jutland Peninsula. The geographical area today covers the large area between the Eider river in the south and the Flensburg Fjord in the north, where it borders Denmark. Northern Schleswig, congruent with the former South Jutland County. The area belonged to the Crown of Denmark until the Prussians and Austrian declared war on Denmark in 1864. Denmark wanted to give away the German speaking Holsten and set the new border at the small river Ejderen. This was a reason for war, did Prussian chancellor Otto von Bismarck conclude, and even proclaimed it as a "holy war". The German chancellor also turned himself to the Emperor of Austria, Franz Joseph I of Austria for help. A similar war in 1848 had got all wrong for the Prussians. With help of both the Austrians and the Danish born General Moltke was the Danish army destroyed or forced to make disordered retreat. And the Prussian - Danish border was moved from the Elbe up in Jutland to the creek Kongeåen.
After the First World War, two referendums decided a new border The northern part went back to Denmark as Nordslesvig (North Slesvig). But the middle and southern part including Schleswig's only city, Flensburg, remained in what now was German hands, rather than Prussian ones. In Denmark the loss of Flensborg caused a political crisis, Påskekrisen or the Easter Crisis, as it happened during the Easter of 1920. But Südschleswig including the city of Flensburg remained as a part of Prussia in the new German republic, the Weimar Republic. Also after the Second World War the area remained as German territory and, with Holstein, formed the new Federal Republic of Schleswig-Holstein as a part of the Federal Republic of Germany (West Germany) in 1948.