The old border
Already from the 1200s, the river Kongeå, together with the river Kolding Å formed the border between the Duchy of Schleswig and the Kingdom of Denmark.
From 1500 and until 1850, the river Kongeå was the customs border between Schleswig and Denmark. Although both areas belonged United Monarchy of Denmark, the payment of customs duties was still obligatory when crossing the border.
In 1851, the customs border was moved south to the Eider and for the first time in centuries, it was now possible to freely cross the the river Kongeå.
After Denmark's defeat by Prussia in the war of 1864, Kongeåen became part of the border between Denmark and Germany once again. It remained so until reunification in 1920, when Southern Jutland became Danish again.