Schleswig–Holstein question

The Schleswig–Holstein question was a complex set of diplomatic and other issues arising in the 19th century from the relations of two duchies, Schleswig (Sønderjylland/Slesvig) and Holstein (Holsten), to the Danish Crown, to the German Confederation, and to each other. Schleswig was part of Denmark during the Viking Age, and became a Danish duchy in the 12th century – legally part of Denmark, but in many ways autonomous.

Source: Wikipedia — Schleswig–Holstein question (CC BY-SA 4.0)