"Denmark to the Eider" was a slogan for the National Liberal Party in the context of the Schleswig-Holstein Question of the mid 1800s. The Eider River in the Jutland peninsula was deemed by the National Liberals to be the border of Denmark. This ultimately led to war with Germany.

The roots of this problem lie with the ownership of the Jutland peninsula. All of the peninsula above the Kongeå River had always been part of the Danish Kingdom. The area between the Kongeå and the Eider River was part of the Duchy of Schleswig, wheras the area between the Eider and the Elbe River was the Duchy of Holstein. It was the relationship between Denmark, the Duchies and Germany that lay at the roots of this problem.

The Duchy of Schleswig was ruled as a fief of Denmark by the Danish King, who was also the Duke. The Duchy of Holstein was German, but had always had a close connection with Denmark with the Danish King sometimes also being the Duke. When the Danish King Frederick VII came to the throne in 1848, he was also Duke of both Schleswig and Holstein. However, Frederick VII did not have any heirs and so the conflict over the territories began.

Several years before Frederick VII came to the throne it was realised that after three marriages, he would never have children and plans were made for his successor. The problem was that Denmark had different inheritence laws to the Duchies and that meant that the successor Prince Christian of Glücksburg could inherit Denmark but not Schleswig-Holstein.

Denmark planned to change the rules on succession and this sparked off a revolt by the German majority in the Duchies supported by Prussia that ended in the First Schleswig War (1848-1851). Denmark was victorious in this war and absorbed the Duchies into the Kingdom. A Second Schleswig War, fought in 1864, ended in complete disaster for Denmark, with Jutland invaded by both Prussia and Austria. 

After the Second Schleswig War, the border was drawn at the Kongeå where it was to stay until 1920. In this year the Treaty of Versailles had allowed for a plebiscite in Schleswig and the northern part of the Duchy voted to join Denmark. There still remains a Danish minority of about 50,000 in South Schleswig, with 10,000 speaking Danish on a day to day basis.