Why did Prussia and Austria declare war on Denmark in 1864?

Prussia and Austria declared war on Denmark in 1864 primarily due to a constitutional crisis involving the duchies of Schleswig and Holstein.12

The conflict was rooted in the Schleswig-Holstein question, a long-standing dispute over whether these territories should be integrated into Denmark or remain associated with the German Confederation. In 1863, the Danish government, under pressure from nationalist movements, enacted a new "November constitution" that effectively annexed Schleswig to Denmark. This action violated previous international agreements—specifically the 1852 London Protocol—which stipulated that Denmark would not tie Schleswig more closely to itself than to Holstein.1234

Prussia and Austria intervened as the upholders of these international agreements, demanding that Denmark rescind the new constitution. When Denmark refused, the two powers launched a military campaign to enforce their demands and resolve the status of the duchies. Beyond the constitutional dispute, the war was also driven by the political ambitions of Otto von Bismarck, who sought to secure the duchies for Prussia.254