Duchy of Holstein

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.

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:

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.