Duchy of Schleswig in the context of "King Christian IX of Denmark"

The House of Oldenburg is a German dynasty whose members rule or have ruled in Denmark, Iceland, Greece, Norway, Russia, Sweden, the United Kingdom, Livonia, Schleswig, Holstein, and Oldenburg. The current kings of Norway and the United Kingdom are patrilineal descendants of the Glücksburg branch of this house.

The dynasty rose to prominence when Count Christian I of Oldenburg was elected King of Denmark in 1448, of Norway in 1450 and of Sweden in 1457. Denmark's current monarch, King Frederik X, belongs to the Glücksburg branch of the House of Oldenburg through his mother, Queen Margrethe II.

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.

Denmark–Norway was a 16th-to-19th-century multi-national and multi-lingual real union consisting of the Kingdom of Denmark, the Kingdom of Norway (including the then Norwegian overseas possessions: the Faroe Islands, Iceland, Greenland, and other possessions), the Duchy of Schleswig, and the Duchy of Holstein. The state also claimed sovereignty over three historical peoples: Frisians, Gutes and Wends. Denmark–Norway had several colonies, namely the Danish Gold Coast, Danish India (the Nicobar Islands, Serampore, Tharangambadi), and the Danish West Indies.

The state's inhabitants were mainly Danes, Norwegians and Germans, and also included Faroese, Icelanders and Inuit in the Norwegian overseas possessions, a Sami minority in northern Norway, as well as other indigenous peoples. The main cities of Denmark–Norway were Copenhagen, Christiania (Oslo), Altona, Bergen and Trondheim, and the primary official languages were Danish and German, but Norwegian, Icelandic, Faroese, Sami and Greenlandic were also spoken locally.

Christian X (Danish: Christian Carl Frederik Albert Alexander Vilhelm; 26 September 1870 – 20 April 1947) was King of Denmark from 1912 until his death in 1947, and the only King of Iceland as Kristján X, holding the title as a result of the personal union between Denmark and independent Iceland between 1918 and 1944.

He was a member of the House of Glücksburg, a branch of the House of Oldenburg, and the first monarch since King Frederick VII born into the Danish royal family; both his father and his grandfather were born as princes of a ducal family from Schleswig. Among his siblings was King Haakon VII of Norway. His son became Frederick IX of Denmark. Among his cousins were King George V of the United Kingdom, Emperor Nicholas II of Russia, and King Constantine I of Greece, while Queen Maud of Norway, was both his cousin and sister-in-law.

Frederick II (Danish: Frederik 2.; 1 July 1534 – 4 April 1588) was King of Denmark and Norway and Duke of Schleswig and Holstein from 1559 until his death in 1588.

A member of the House of Oldenburg, Frederick began his personal rule of Denmark-Norway at the age of 24. He inherited capable and strong realms, formed in large by his father after the civil war known as the Count's Feud, after which Denmark-Norway saw a period of economic recovery and of a great increase in the centralised authority of the Crown.

The Duchy of Holstein (German: Herzogtum Holstein; Danish: Hertugdømmet Holsten) was the northernmost state of the Holy Roman Empire, located in the present German state of Schleswig-Holstein. It originated when King Christian I of Denmark had his County of Holstein-Rendsburg elevated to a duchy by Emperor Frederick III in 1474. Members of the Danish House of Oldenburg ruled Holstein – jointly with the Duchy of Schleswig – for its entire existence.

From 1490 to 1523 and again from 1544 to 1773 the Duchy was partitioned between various Oldenburg branches, most notably the dukes of Holstein-Glückstadt (identical with the Kings of Denmark) and Holstein-Gottorp. The duchy ceased to exist when the Kingdom of Prussia annexed it in 1866 after the Austro-Prussian War.

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.

The First Schleswig War (German: Schleswig-Holsteinischer Krieg), also known as the Schleswig-Holstein uprising (German: Schleswig-Holsteinische Erhebung) and the Three Years' War (Danish: Treårskrigen), was a military conflict in southern Denmark and northern Germany rooted in the Schleswig–Holstein question: who should control the Duchies of Schleswig, Holstein and Lauenburg, which at the time were ruled by the king of Denmark in a personal union. Ultimately, the Danish side proved victorious with the diplomatic support of the great powers, especially Britain and Russia, since the duchies were close to an important Baltic seaway connecting both powers.

While Schleswig had been predominantly Danish with a German elite concentrated in the cities and estates, modernisation brought extensive German influence. German became the language of administration, education, and the church, creating an environment in which it was omnipresent and speaking Danish offered no advantages. Due to top-down pressure, many Danish South Schleswigers gradually adopted German in their daily lives. This linguistic shift led to the extinction of the two Danish South Schleswig dialects, Fjoldemål and Angeldanish – first in favour of Low German, and later High German. Over time, these now Low German-speaking South Schleswigers became fully Germanized, came to identify with the German question, and sought to separate Schleswig, along with Holstein, from Denmark entirely. Therefore, by the outbreak of the war, Schleswig was divided: a Danish-speaking North and a Low German-speaking South, while Holstein and Lauenburg were entirely ethnically German.

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.