Official Languages: Low German, Danish
Established: 1474 AD/CE
Disestablished: 1864 AD/CE
The northern border of Holstein along the Eider River had already formed the northern border of the Carolingian Empire, after Emperor Charlemagne upon the Saxon Wars reached an agreement with King Hemming of Denmark in 811. The lands of Schleswig beyond the river remained a fief of the Danish Crown, while Holstein became an integral part of East Francia, the Kingdom of Germany and the Holy Roman Empire.
Adolf VIII, the last Count of Holstein-Rendsburg and Duke of Schleswig had died without heirs in 1459. As Schleswig had been a Danish fief, it had to fall back to King Christian I of Denmark, who, himself a nephew of Adolf, also sought to enter into possession of Holstein. He was backed by the local nobility, who supported the continued common administration of both lands and by the 1460 Treaty of Ribe proclaimed him as the new Count of Holstein.
Nevertheless, the comital Holstein lands south of the Eider River officially remained a mediate fief held by the Ascanian dukes of Saxe-Lauenburg. In 1474 Emperor Frederick III conferred Imperial immediacy to Christian by elevating him to a Duke of Holstein.
In 1544, the Duchies of Schleswig and Holstein were partitioned in three parts between Christian's grandson Christian III of Denmark and his two younger half-brothers (who had to renounce the Danish throne), as follows:
- The royal part, held by Christian III and his successors (identical with the Kings of Denmark). From 1648 the royal parts of Schleswig and Holstein were administered out of Glückstadt and became known as the Duchy of Schleswig-Holstein-Glückstadt. Before 1773 its Holstein territory consisted of the following Ämter: Rendsburg, South Dithmarschen, Steinburg, Segeberg, and Plön.
- The Duchy of Schleswig-Holstein in Haderslev, held by Duke Hans the Elder. Hans had no issue and after his death in 1580, his territories were divided among his brothers.
- The Duchy of Schleswig-Holstein in Gottorp, held by Duke Adolf and his successors.
In addition, significant parts of Holstein were jointly administered by the Dukes of Holstein-Glückstadt and the Dukes of Holstein-Gottorp, mainly on the Baltic Sea coast.
In 1640, the County of Holstein-Pinneberg, whose ruling house was extinct, was merged in the royal part of the Duchy of Holstein.
In 1713, during the Great Northern War, the estates of the Dukes of Schleswig-Holstein-Gottorp in Schleswig including Schloss Gottorf were conquered by royal Danish troops. In the 1720 Treaty of Frederiksborg, Duke Charles Frederick of Schleswig-Holstein-Gottorp ceded them to his liege lord the Danish crown.
His remaining territories formed the Duchy of Holstein-Gottorp, administered from Kiel. In 1773, Charles Frederick's grandson, Paul, Emperor of Russia finally gave his Holstein lands to the Danish king, in his function as Duke of Holstein, in exchange for the County of Oldenburg, and Holstein was reunited as a single state.
With the dissolution of the Holy Roman Empire in 1806, the Duchy of Holstein gained sovereignty.
After the 1815 Congress of Vienna, the Duchy of Holstein became a member of the German Confederation, resulting in several diplomatic and military conflicts about the so-called Schleswig-Holstein Question. Denmark defended its rule over Holstein in the First Schleswig War of 1848-51 against the Kingdom of Prussia. However, in the Second Schleswig War (1864) Prussian and Austrian troops conquered Schleswig. Christian IX of Denmark had to renounce both Schleswig and Holstein in the Treaty of Vienna (1864) on October 30. At first placed under joint rule in a condominium, Prussia and Austria then assumed administration of Schleswig and Holstein, respectively, under the Gastein Convention of August 14, 1865. However, tensions between the two powers culminated in the Austro-Prussian War of 1866. Following the Peace of Prague (1866), the victorious Prussians annexed both Schleswig and Holstein by decree of December 24, 1866, and later established the unified Province of Schleswig-Holstein.