Southern Schleswig

Southern Schleswig 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, forms the southernmost part of Denmark. The area belonged to the Crown of Denmark until Prussia and Austria 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. Prussian chancellor Otto von Bismarck concluded that this justified a war, and even proclaimed it a "holy war". He also turned to the Emperor of Austria, Franz Joseph I of Austria for help. A similar war in 1848 had gone poorly for the Prussians. With Prussia's modern weapons and the help from both the Austrians and the Danish-born General Moltke, the Danish army was destroyed or forced to make a disorderly 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 reverted to Denmark as Nordslesvig . But the middle and southern part, including Schleswig's only city, Flensburg, remained in what, since the unification of Germany, had become German hands. In Denmark, the loss of Flensborg caused a political crisis, Påskekrisen or the Easter Crisis, as it happened during the Easter of 1920. After the Second World War the area remained as German territory and, with Holstein, formed the new state of Schleswig-Holstein as a part of the Federal Republic of Germany in 1948.

Wikipedia