Holstein in the context of "Jutland Peninsula"

The history of Denmark as a unified kingdom began in the 8th century, but historic documents describe the geographic area and the people living there—the Danes—as early as 500 AD. These early documents include the writings of Jordanes and Procopius. With the Christianization of the Danes c. 960 AD, it is clear that there existed a kingship. King Frederik X can trace his lineage back to the Viking kings Gorm the Old and Harald Bluetooth from this time, thus making the Monarchy of Denmark the oldest in Europe. The area now known as Denmark has a rich prehistory, having been populated by several prehistoric cultures and people for about 12,000 years, since the end of the last ice age.

Denmark's history has particularly been influenced by its geographical location between the North and Baltic seas, a strategically and economically important placement between Sweden and Germany, at the center of mutual struggles for control of the Baltic Sea (dominium maris baltici). Denmark was long in disputes with Sweden over control of Skånelandene and with Germany over control of Schleswig (a Danish fief) and Holstein (a German fief).

Kiel (/kiːl/ KEEL; German: [kiːl] ) is the capital and most populous city in the northern German state of Schleswig-Holstein. With a population of around 250,000, it is Germany's largest city on the Baltic Sea. It is located on the Kieler Förde inlet of the Bay of Kiel and lies in the southeast of the Jutland Peninsula, on the mouth of the Schwentine River, approximately 90 kilometres (56 mi) northeast of Hamburg. The world's busiest artificial waterway, the Kiel Canal, has a terminus in Kiel's Holtenau district. This canal connects the Baltic to the North Sea, with its other end in Brunsbüttel. Most of Kiel is part of Holstein. The boroughs north of the Schwentine also belong to Wagria, while those north of the Kiel Canal are historically part of Southern Schleswig.

Originally a small settlement, Kiel was granted city rights in 1242. Over the centuries, it developed as an important port and trading hub, particularly due to its strategic location on the Kieler Förde inlet. Kiel became part of the Duchy of Holstein, the northernmost state of the Holy Roman Empire. From 1773 to 1864, it was ruled in personal union by the king of Denmark. In 1864, after the Second Schleswig War, the city was incorporated into the Kingdom of Prussia. During the 20th century, Kiel played a significant role in both World War I and World War II, with its naval significance and shipbuilding industry. Following World War II, it became part of West Germany and later the unified Germany.

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 Eider (German: Eider [ˈaɪdɐ] ; Danish: Ejderen; Latin: Egdor or Eidora) is the longest river in the German state of Schleswig-Holstein. The river starts near Bordesholm and reaches the southwestern outskirts of Kiel on the shores of the Baltic Sea, but flows to the west, ending in the North Sea. The lower part of the Eider was used as part of the Eider Canal until that canal was replaced by the modern Kiel Canal.

In the Early Middle Ages the river is believed to have been the border between the related Germanic tribes, the Jutes and the Angles, who along with the neighboring Saxons crossed the North Sea from this region during this period and settled in England. During the High Middle Ages the Eider was the border between the Saxons and the Danes, as reported by Adam of Bremen in 1076. For centuries it divided Denmark and the Holy Roman Empire. Today it is the border between Schleswig, Holstein and Eiderland, the northern and southern parts, respectively, of the modern German state of Schleswig-Holstein.

The Obotrites (Latin: Obotriti, Abodritorum, Abodritos) or Obodrites, also spelled Abodrites (German: Abodriten), were a confederation of medieval West Slavic tribes within the territory of modern Mecklenburg and Holstein in northern Germany (see Polabian Slavs). For decades, they were allies of Charlemagne in his wars against the Germanic Saxons and the Slavic Veleti. The Obotrites under Prince Thrasco defeated the Saxons in the Battle of Bornhöved (798). The still-Pagan Saxons were dispersed by the emperor, and the part of their former land in Holstein north of Elbe was awarded to the Obotrites in 804, as a reward for their victory. This however was soon reverted through an invasion of the Danes. The Obotrite regnal style was abolished in 1167, when Pribislav was restored to power by Duke Henry the Lion, as Prince of Mecklenburg, thereby founding the Germanized House of Mecklenburg.