The Eider Policy, or in Danish "Ejderpolitikken", refers to the National Liberal policy implemented in Denmark around 1840. The Danish politician Orla Lehmann was the front figure of this movement.

The main purpose of this policy was to define the Danish state and its borders. The National Liberal policy wanted the border to be defined by the river "Ejderen" or "Eider" in the Jutland peninsula. The river was considered as a natural border between Denmark and Germany. It is from this specific policy that the slogan "Danmark til Ejderen" was fabricated.

The Duchies of Holstein, Schleswig and Lauenburg became the issue in dispute in the Danish delineation of frontiers. The Danish king and the Conservatives where against the National Liberal wishes for the border. They wanted to preserve the United Monarchy.

The Eider Policy was accepted in 1863 and in 1864, a war for the territories became a reality. Denmark lost the war. Therefore, the Eider Policy ended up helping Bismarck unite Germany into one nation instead of its actual purpose.

The border has been the object of a lot of disagrements, wars and crisis over the years. In the aftermath of World War 1, the Treaty of Versailles 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.