Schleswig's history is inseparably combined with the German-Danish history. The city and Gottorf Castle have played a significant role in the change of political events for a long time. Due to the increasing presence of the Danish dukes at Gottorf Castle from 1544 to 1713, Schleswig became the residence city of the state Schleswig-Holstein-Gottorf. During that time, the castle was expanded several times thereby becoming a symbol of the increasing power of the Danish dukes. Starting in 1840, the German-Danish conflict became the dominant topic in the city. Most citizens supported the German side and, during the singer festival in 1840, the unofficial anthem of Schleswig-Holstein, "Wanke nicht, mein Vaterland" meaning 'do not falter, my fatherland', was created.  This was also the first time a banner with the colours blue, white and red was presented which to date are the national colours of Schleswig-Holstein.

In 1848, the First Schleswig War broke out originating with the German minded population of Schleswig and Holstein against the reign of the Danish king in the duchies. In April, the battle at Schleswig was fought which resulted in the banishment of Danish troops in Schleswig but by 1851, at the end of the military conflict, the Danish kingdom defeated the German minded movement in Schleswig-Holstein. As a result, the duchies Schleswig and Holstein were joined by personal union with the Danish monarchy. Together with Schleswig, the duchy of Schleswig remained joined with Denmark as a Danish fief, whereas Holstein remained part of the German Confederation. The ducal authorities within the kingdom of Denmark were reorganised and Schleswig lost all ducal government authorities.

In the Second Schleswig War (1864), Austrian and Prussian troops defeated the Danish troops at the Battle of Dybbøl. In August 1865, Prussia took the duchy of Schleswig and the duchy of Saxe-Lauenburg while Austria took Holstein. In 1866, Prussia occupied Holstein, which became the official pretext for the Austro-Prussian War in which Prussia annexed Holstein and combined all three territories thus creating the Prussian province Schleswig-Holstein in 1867.

In the First World War, Denmark remained neutral. After a referendum in 1920, northern Schleswig became part of Denmark. The central and southern part of Schleswig remained in Germany. Today, this drawn border still forms the border demarcation.
 

Remains of the partly turbulent period of the Danes and Germans have not only left their visible traces in Schleswig but also throughout the entire region. Gottorf Castle certainly is the clearest indication of the Danish influenced period. By now, the castle is the location of the national museums of Schleswig-Holstein, hosting the most important collections of art, culture and archaeology in Northern Europe.

The oldest banner in blue, white and red from 1845 is exhibited in Schleswig's city museum. Its predecessor was first introduced by the German minded population during the singing festival in 1840.

During the Second Schleswig War, the Battle of Idstedt took place in 1850 at which 37,000 Danish soldiers defeated the Schleswig-Holstein troops before they eventually lost the dominion over Schleswig and Holstein in 1864. Today, you can still find old documents, paintings, pictures and illustrations of this political and military era at the Idstedt-Gedächtnishalle (Idstedt memorial hall ).

In addition, the German-Danish history has influenced and shaped the contemporary language, e.g. endings of some village names

• - by: original meaning “farm”, then: a collection of farms = small village
• - rup: small village, rural settlement
• - thorp: term for a small neighbouring settlement
• - holm: term for a small island

The Danish influence is not only visible in art and culture but also in politics. The Danish population of south Schleswig is a minority group and is subject to the protection of minorities. The Danish government also guarantees similar minority rights to the German minority in Denmark.

According to official sources, approximately 50,000 people belong to this minority today. One study at Hamburg University even suggested twice as many. The South Schleswig Voters' Association (SSW) is the political representation of the Danish minority. In the Schleswig-Holstein Landtag (parliament), the five per cent hurdle was suspended in favour of the SSW.

Many members of the Danish population are organised in the Südschleswigschen Verein (South Schleswig Association). The Danish secondary school A. P. Møller Skolen, established in 2008 in the Schleswig district 'Auf der Freiheit' belongs to the sponsorship of the Dansk Skoleforening for Sydslesvig and is a school for members of the Danish minority.

The teaching language is Danish and the graduation is accepted both in Germany and Denmark. The school serves as a complement to the two existing Danish primary schools in Schleswig. It was naugurated by Queen Margrethe II of Denmark in 2008. This 60 million euro building was a present to the Danish minority from the private, non-profit A. P. Møller Foundation.