First Schleswig War in the context of "Saxe-Lauenburg"

Schleswig-Holstein (/ˈʃlɛswɪɡ ˈhoʊlstaɪn/; German: [ˌʃleːsvɪç ˈhɔlʃtaɪn] ; Danish: Slesvig-Holsten [ˌsle̝ːsvi ˈhʌlˌste̝ˀn]; Low German: Sleswig-Holsteen; North Frisian: Slaswik-Holstiinj; occasionally in English Sleswick-Holsatia) is the northernmost of the 16 states of Germany, comprising most of the historical Duchy of Holstein and the southern part of the former Duchy of Schleswig. Its capital city is Kiel; other notable cities are Lübeck and Flensburg. It covers an area of 15,763 km (6,086 sq mi), making it the 5th smallest German federal state by area (including the city-states). Historically, the name can also refer to a larger region, containing both present-day Schleswig-Holstein and the former South Jutland County (Northern Schleswig; now part of the Region of Southern Denmark) in Denmark.

Schleswig, known as South Jutland at the time, had been under Danish control since the fifth century. In the 12th century, however, it became a duchy within Denmark due to infighting in the Danish royal house. It bordered Holstein, which was a part of the Holy Roman Empire. Beginning in 1460, the King of Denmark ruled both Schleswig and Holstein as their duke. Schleswig was still part of Denmark, while Holstein remained part of the Holy Roman Empire. In the 19th century, both Danes and Germans believed they had a rightful claim to Schleswig-Holstein. While the population of Holstein and Lauenburg was entirely ethnically German, Schleswig had been predominantly Danish until the late 1700s and early 1800s, when extensive German influence led to the Germanization of South Schleswig. The resulting long-term political and territorial dispute was known as the Schleswig-Holstein Question. In 1848, Denmark tried to formally annex the area. Prussia responded by invading, thus beginning the First Schleswig War, which ended in a victory for Denmark. But in the Second Schleswig War (1864), Prussia and Austria won, and the territory was absorbed into Prussia in 1867. After the German defeat in World War I the Allies required that the question of sovereignty over the territory be submitted to plebiscites (the 1920 Schleswig plebiscites). They resulted in the return of the Danish-speaking North Schleswig to Denmark. The area was subsequently renamed South Jutland, restoring the historical name used before it was changed to Schleswig by Gerhard III of the House of Schauenburg. After World War II, Schleswig-Holstein took in over a million refugees.

The Duchy of Schleswig (/ˈʃlɛswɪɡ/; German: [ˈʃleːsvɪç]; Danish: Hertugdømmet Slesvig; German: Herzogtum Schleswig; Low German: Hartogdom Sleswig; North Frisian: Härtochduum Slaswik) was a duchy in Southern Jutland (Sønderjylland) covering the area between about 60 km (35 miles) north and 70 km (45 mi) south of the current border between Germany and Denmark. The territory has been divided between the two countries since 1920, with Northern Schleswig in Denmark and Southern Schleswig in Germany. The region is also called Sleswick in English.

Unlike Holstein and Lauenburg, Schleswig was never a part of the German Confederation. Schleswig was instead a fief of Denmark, and its inhabitants spoke Danish, German, and North Frisian. Both Danish and German National Liberals wanted Schleswig to be part of a Danish or German national state in the 19th century. A German uprising in March 1848 caused the First Schleswig War which ended in 1852. The Second Schleswig War (1864) ended with the three duchies being governed jointly by Austria and Prussia. In 1866, they became a part of Prussia.

The Second Schleswig War (Danish: Den anden slesvigske krig; German: Deutsch-Dänischer Krieg or German Danish War), also sometimes known as the Dano-Prussian War or Prusso-Danish War, was the second military conflict over the Schleswig–Holstein question of the 19th century. The war began on 1 February 1864, when Prussian and Austrian forces crossed the border into the Danish fief Schleswig. Denmark fought troops of the Kingdom of Prussia and the Austrian Empire representing the German Confederation.

Like the First Schleswig War (1848–1852), it was fought for control of the duchies of Schleswig, Holstein, and Lauenburg. Succession disputes concerning the duchies arose when Frederick VII, King of Denmark died without an heir acceptable to the German Confederation. The war started after the passing of the November Constitution of 1863, which tied the Duchy of Schleswig more closely to the Danish kingdom, which was viewed by the German side as a violation of the London Protocol.