The area's traditional significance lies in the transfer of goods between the North Sea and the Baltic Sea, connecting the trade route through Russia with the trade routes along Rhine and the Atlantic coast (see also Kiel Canal).
Roman sources place the homeland of the Jute tribe north of the river Eider and that of the Angles to its south who in turn abutted the neighboring Saxons. Towards the end of the Early Middle Ages, Schleswig formed part of the historical Lands of Denmark as Denmark unified out of a number of petty chiefdoms in the 8th to 10th centuries (The heyday of the Viking incursions).
During the early Viking Age, Haithabu - Scandinavia's biggest trading centre - was located in this region which is also the location of the Danewerk. This construction, and in particular its great expansion around 737 has been interpreted as an indication of the emergence of a unified Danish state.
In May 1931 scientists of the National Museum of Denmark announced the finding of eighteen Viking graves with eighteen men in them. The discovery came during excavations in Schleswig. The skeletons indicated that the men were bigger proportioned than twentieth century Danish men. Each of the graves was turned east to west. It was surmised that the bodies were entombed in wooden coffins originally, but only the iron nails remained.
During the 10th century, ownership over the region between the Eider River and the Danevirke became a matter of dispute between the Holy Roman Empire and Denmark, resulting in several wars. In 974, Otto II, Holy Roman Emperor concluded a successful campaign by erecting a fortress, which was however razed by Sweyn Forkbeard in 983.
Denmark again attempted to integrate Schleswig in 1864, but the German Confederation defeated the Danes in the Second War of Schleswig. Prussia and Austria respectively assumed administration of Schleswig and Holstein under the Gastein Convention of 14 August 1865. However, tensions between the two powers culminated in the Austro-Prussian War of 1866, in which victorious Prussia annexed Schleswig and Holstein, creating the province of Schleswig-Holstein.
Nowadays, both parts cooperate as a Euroregion, despite the national frontier dividing the former duchy.
Germans strongly objected to the Danish use of "Sønderjylland". "Olsen's Map", published by the Danish cartographer Olsen in the 1830s and using that term, aroused a storm of protests by German inhabitants. The name "Schleswig", which had a long previous history with no special political connotations, assumed a clear German nationalist character in the 19th century - especially when included in the combined term "Schleswig-Holstein". A central element of the German nationalistic claim was the insistence upon Schleswig and Holstein being a single, indivisible entity. Since Holstein was legally part of the German Confederation and ethnically entirely German with no Danish population, use of that name implied that both provinces should belong to Germany and their connection with Denmark weakened or altogether severed.
For their part, Danes had no objection to the use of "Schleswig" as such, and in fact the name had a commonly-used Danish version "Slesvig". "Sønderjylland" was an older term, hardly used between the 16th and 19th centuries. However, its revival and widespread use in the 19th Century had a clear Danish nationalist connotation of laying a claim to the territory and objecting to the German claims.
The naming dispute was resolved with the 1920 plebiscites and partition, each side applying its preferred name to the part of the territory remaining in its possession - though both terms can in principle still refer to the entire region. Northern Schleswig was after the 1920 plebiscites officially named South Jutlandic districts (de sønderjyske landsdele), while Southern Schleswig became a part of German Schleswig-Holstein.