For other uses of the word, see Holstein

Holstein (Hol-shtayn) is the southern part of Schleswig-Holstein in Germany, between the rivers Elbe, Eider and the Schlei firth. The capital of Holstein is Kiel.

1111-1474 an Imperial (Reichsunmittelbar) County of the Holy Roman Empire, although some times occupied by Denmark, and thereafter an Imperial Duchy until the dissolution of the Empire in 1806. From the 15th century, Holstein was, along with the Danish Duchy of Schleswig, inherited by the Kings of Denmark. However, the two duchies were both divided up, with part under the control of the Kings of Denmark, and part under the Dukes of Holstein-Gottorp, a cadet line of the family. The Duke of Holstein-Gottorp was forced to give up his lands in Schleswig to the Kings of Denmark following the Great Northern War in 1720, but he retained his lands in Holstein until 1773. The Danes were eager to round out their possessions, however, especially after the Duke of Holstein-Gottorp became Emperor of Russia in 1762 as Peter III and was planning an attack on Denmark to recover the lost Gottorp lands in Schleswig. Although Peter was soon overthrown by his wife, Catherine the Great, the Danes determined to rid themselves of this problem. In 1773, they exchanged the County of Oldenburg for the Gottorp lands in Holstein, bringing all of Holstein under their control.

From 1806 to 1864 the Duchy of Holstein was part of the German Confederation, though in personal union with Denmark (the King of Denmark being also Duke of Holstein). Following the death of King Frederick VII of Denmark in 1863, the inheritance of Schleswig and Holstein was disputed. The new king, Christian IX, made his claim to the Danish throne through a female line. The Prince of Augustenburg, a minor princeling from another line of the family, claimed the Duchies, and soon the German Confederation, led by Prussia and Austria, went to war with Denmark, quickly defeating it in 1864 and forcing it to cede the duchies. The duchies were not, however, given to the Prince of Augustenburg, and in 1865 an arrangement was worked out between Prussia and Austria where the Austrians occupied and administered Holstein, while the Prussians did the same in Schleswig. This arrangement came to an end with the Austro-Prussian War of 1866, which resulted in Schleswig and Holstein both being incorporated within Prussia.