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What caused the Danish War of 1864 between Prussia and Denmark?

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Historical map showing the unification of Germany by Prussia in the late 19th century. The map highlights in green the Kingdom of Prussia as of 1866, and in yellow the additional territories that formed the German Empire by 1871-1918, encompassing most of north-central Europe. Neighboring countries such as France, Belgium, the Netherlands, Denmark, Sweden, Russia, Austria-Hungary, and Switzerland are labeled, with major regions and states within the empire clearly marked. — Prussia The unification of Germany by Prussia, which brought most of north-central Europe into one kingdom.

The Danish War of 1864, also known as the German-Danish War, was primarily caused by a long-standing constitutional and nationalist dispute over the status of the duchies of Schleswig and Holstein.12

While these duchies were ruled by the Danish king, they had distinct populations and legal statuses. German nationalists sought to keep the duchies associated with the German Confederation, while Danish nationalists—the Eider-Danes—aimed to incorporate Schleswig directly into Denmark by establishing the Eider River as the national boundary.12

Tensions escalated in 1863 when the new Danish king, Christian IX, signed a joint constitution that tied Schleswig more closely to Denmark, violating the terms of the London Protocol of 1852. Prussia and Austria intervened as upholders of the 1852 protocol, demanding that Denmark rescind the constitution. When Denmark refused, the two powers invaded in 1864. For Otto von Bismarck, the conflict also served as a strategic opportunity to assert Prussian influence and potentially annex the duchies.2345