First Schleswig War

The First Schleswig War, also known as the Schleswig-Holstein Uprising and the Three Years' War, was a military conflict in southern Denmark and northern Germany rooted in the Schleswig-Holstein Question, contesting the issue of who should control the Duchies of Schleswig, Holstein and Lauenburg, who at the time were ruled by the king of Denmark in a personal union. Ultimately, the Danish side proved victorious with the diplomatic support of the great powers, especially Britain and Russia, since the duchies were close to an important Baltic seaway connecting both powers.

The population was majority Danish in Northern Schleswig while in the rest of Schleswig and in Holstein and Lauenburg it was majority German.

Source: Wikipedia