The First Schleswig War, also known as the Schleswig-Holstein Uprising or the Three Years' War, began in March 1848 and was rooted in the Schleswig-Holstein Question, a dispute over who should control the Duchies of Schleswig, Holstein, and Lauenburg. These territories were ruled by the King of Denmark in a personal union, but the populations of these regions were divided along ethnic lines, with the northern part of Schleswig having a Danish majority, while the rest of Schleswig, as well as Holstein and Lauenburg, had a predominantly German population.
Tensions erupted in March 1848 when the German-speaking population of Schleswig, Holstein, and Lauenburg rebelled against Danish rule. They established a provisional government and formed an army to fight for their cause. As Holstein and Lauenburg were member states of the German Confederation, the Confederation supported the rebellion, transforming the conflict into a federal war (Bundeskrieg). Prussia, a leading power in the German Confederation, played a prominent role by providing most of the German troops to support the uprising.
The first phase of the war was intense, but hostilities were temporarily halted in August 1848 with the signing of the armistice of Malmö. This truce provided a brief period of peace but failed to resolve the fundamental issues, and in February 1849, fighting resumed when the Danish launched a renewed offensive against the rebels.
By the summer of 1850, the tide had turned against the rebels. Prussia, facing pressure from the major European powers, was forced to withdraw its support for the Schleswig-Holstein forces. This left the rebels isolated, and by April 1, 1851, the Schleswig-Holstein army was officially disbanded, marking the end of their struggle for independence.
The conflict concluded with the signing of the London Protocol in 1852. This agreement, backed by the great powers, including Britain and Russia, confirmed the Danish King as the Duke of Schleswig, Holstein, and Lauenburg but stipulated that these duchies would remain independent from Denmark proper. Despite the Danish victory, the underlying issues remained unresolved, setting the stage for future conflicts over these territories.