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. Although Hitler occupied Denmark between 9 April 1940 and the late evening of 4 May 1945 (with exception of the Baltic island of Bornholm, which was occupied by the Soviet Red Army between 9 May 1945 and 5 April 1946), he never annexed any part of Denmark. Perhaps Hitler told the truth, when he wrote "The new Germany must grow toward the east". But the Belgium cities of Eupen and Malmedy was re-annexed to the Third Reich in the summer of 1940. After the First World War had they been given to Belgium as a part of the Versailles Treaty.
The Schleswig lands north of the Eider river and the Bay of Kiel had been a fief of the Danish Crown since the Early Middle Ages. The southern Holstein region belonged to Francia and later to the Holy Roman Empire, it was however held as an Imperial fief by the Danish kings since the 1460 Treaty of Ribe.
The Schleswig-Holstein Question at first culminated in the course of the Revolutions of 1848, when from 1848 to 1851 revolting German-speaking National liberals backed by Prussia fought for the separation of Schleswig and Holstein from Denmark in the First Schleswig War. Though the status quo was restored, the conflict lingered on and on 1 February 1864 the Prussian and Austrian troops crossed the Eider sparking off the Second Schleswig War, after which Denmark had to cede Schleswig and Holstein according to the Treaty of Vienna. After the Austro-Prussian War of 1866, victorious Prussia took control over all Schleswig and Holstein but was obliged by the Peace of Prague to hold a referendum in predominantly Danish-speaking Northern Schleswig, which it never did.
After the German defeat in World War I the Schleswig Plebiscites were decreed by the 1919 Treaty of Versailles, in which the present-day German-Danish border was drawn taking effect on 15 June 1920, dividing Schleswig in a southern and northern part and leaving a considerable Danish and German minority on both sides 
Southern Schleswig is part of the German state (Bundesland) of Schleswig-Holstein, therefore its denotation as Landesteil Schleswig. It does not however form an administrative entity, but consists of the districts (Landkreise) of Schleswig-Flensburg, Nordfriesland, the urban district (Kreisfreie Stadt) of Flensburg and the northern part of Rendsburg-Eckernförde.
Beside Standard German, Low Saxon dialects (Schleswigsch) are spoken, as well as Danish (Standard Danish or South Schleswig Danish) and its South Jutlandic variant, furthermore North Frisian in the west. Danish and North Frisian are official minority languages. Many of the inhabitants who only speak German and not Danish do not consider the region any different from the rest of Schleswig-Holstein. This notion is disputed by those defining themselves as Danes, South Schleswigans or Schleswigans, particularly historians and people organised in the institutions of the Danish minority of Southern Schleswig, such as the South Schleswig Voter Federation. Many of the Last names found in the region are very often of Scandinavian or Danish form, with the -sen endings like Petersen.
- Lars Henningsen: Sydslesvigs danske historie, Flensborg 2013.
- Lars Henningsen: Zwischen Grenzkonflikt und Grenzfrieden, Flensburg 2011 (as pdf document)
- Karen Margrethe Pedersen: Dansk sprog i Sydslesvig: det danske sprogs status inden for det danske mindretal i Sydslesvig, Institut for grænseregionsforskning Aabenraa 2000
- Kathrin Sinner: Schleswig-Holstein - das nördliche Bundesland: Räumliche Verortung als kulturelles Identitäskonstruk, page 86
- Sønderjylland A-Å, Aabenraa 2011, page 364
- German, Troels Fink: "Geschichte des schleswigschen Grenzlandes" Publisher: Munksgaard, Copenhagen 1958, pages 178-192.
- Region Sønderjylland-Schleswig: Politische Entwicklungen im Mittelalter
- Region Sønderjylland-Schleswig: Nationale Entwicklung im 19. Jahrhundert (German)
- vimu - Det Virtuelle Museum: Genforening (Danish)
- vimu - Das Virtuelle Museum: Volksabstimmung (German)
- Region Sønderjylland-Schleswig: Sprachen und Dialekte südlich der Grenze(German)