Coat of arms of Schleswig in the context of "Schleswig"

Southern Schleswig (German: Südschleswig or Landesteil Schleswig, Danish: Sydslesvig; North Frisian: Söödslaswik) 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 General Moltke, the Danish army was destroyed and forced to make a disorderly retreat. The Prussian-Danish border was then moved from the Elbe up in Jutland to the Kongeåen creek.

After the First World War, two referendums decided a new border.The northern part reverted to Denmark as Nordslesvig (North Slesvig). But the middle and southern part, including Schleswig's only city, Flensburg, remained in Germany. In Denmark, the loss of Flensborg sparked a political crisis, Påskekrisen or the Easter Crisis 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 (West Germany) in 1948.