How did the Schleswig-Holstein dispute lead to war between Prussia and Denmark?

The Schleswig-Holstein dispute originated from conflicting succession laws and nationalist tensions regarding the status of the two duchies, which were ruled by the Danish king but held deep ties to Germany. While the duchies were historically associated, Danish nationalists sought to incorporate Schleswig into Denmark, while German nationalists aimed to keep the duchies united and associated with the German Confederation.12

The path to war intensified in 1863 when the Danish government, under King Christian IX, signed a new constitution that attempted to integrate Schleswig more closely into the Danish state. This move violated previous international agreements, specifically the London Protocol of 1852, which had guaranteed the autonomy of the duchies.23

Outraged by this violation, Prussia and Austria intervened as upholders of the protocol. After Denmark refused to rescind the constitution, the two powers declared war in 1864. The resulting German-Danish War led to a swift Danish defeat and the cession of the duchies to Prussia and Austria, setting the stage for further conflict between the two victors.234