Duchy of Holstein

The Duchy of Holstein was the northernmost state of the Holy Roman Empire, situated between the Eider River and the Elbe in what is now Schleswig-Holstein. Its northern border along the Eider had separated the Carolingian empire from Denmark since 811. It originated in 1474, when Emperor Frederick III elevated the County of Holstein-Rendsburg to a duchy for King Christian I of Denmark, who had inherited the county after the death of Adolf VIII in 1459. Holstein was ruled by the Danish House of Oldenburg in personal union with the Duchy of Schleswig, but unlike Schleswig it remained an imperial fief. From 1490 to 1523 and again from 1544 to 1773 the duchy was partitioned among Oldenburg branches, notably the Holstein-Glückstadt and Holstein-Gottorp lines. After the Napoleonic Wars Holstein joined the German Confederation while remaining under the Danish crown. The dispute over Schleswig and Holstein led to the Second Schleswig War, and in 1866 Prussia annexed the duchy after the Austro-Prussian War.