The border between Denmark and Germany has its origin in the Ribe letter from 1460, where The Holstein knights swore their loyalty to Denmark’s Christian I.
Denmark stretched to the Eider since olden times, and some of the land in Southern Jutland was owned by noble families. The Holstein knights swore their loyalty to Denmark’s Christian I in the so-called Ribe Letter, dated 5 March 1460, in which they also declared that the Duchies were to remain united from that day onwards.
With the 1849 Constitution, Denmark wanted Denmark and the duchies to have the same legislation. This led to the First Schleswig War, i.e. the Three Years’ War that lasted from1848-1850. While Denmark won the war, the problem was not resolved. This led to the Second Schleswig War in 1864, whereby Denmark lost Schleswig-Holstein.
The new border, i.e. the Kongeå River Border, came to run south of Ribe, which had more or less been in Danish hands since the 13th century.
When Germany lost WW1, 1914-1918, Southern Jutlanders had to vote on their affiliation. While 74.9 % voted in favour of Denmark in Zone 1, only 19.2 % cast this vote towards the south in Zone 2.
On 15 June 1920, Denmark overtook governmental control of North Schleswig, and on 10 July 1920, Christian X crossed the border on horseback.