The Schleswig–Holstein question (German: Schleswig-Holsteinische Frage; Danish: Spørgsmålet om Sønderjylland og Holsten) 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. Denmark repeatedly tried to fully reintegrate the Duchy of Schleswig into the Danish kingdom. Holstein, just on the other side of the Danish border from Schleswig, was in the Middle Ages a fief of the Holy Roman Empire. From 1460 on, the two had been ruled together by a common Duke, who in practice was also the King of Denmark. The Treaty of Ribe, agreed to by the Danish King in order to gain control of both states, seemed to indicate that Schleswig and Holstein were to remain united, though that interpretation was later challenged.
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