The Second Schleswig War was the second military conflict due to the Schleswig-Holstein Question. The war began on February 1, 1864 when Prussian forces crossed the border into Schleswig. The war ended on October 30, 1864 with the Treaty of Vienna (1864) causing Denmark's cession of the Duchies of Schleswig, Holstein and Lauenburg to Prussian and Austrian administration, respectively.

Other names by which the war is known include the Danish-Prussian War, the German-Danish War (German: Deutsch-DA nischer Krieg), the Prusso-Danish War, the War of 1864, and the Schleswig-Holstein War of Succession.

The war took place in 1864 between Denmark on the one side and Prussia and Austria on the other side. Like the First Schleswig War (1848A 51), it was fought for control of the duchies because of succession disputes concerning the duchies of Holstein and Lauenburg when the Danish king died without an heir acceptable to the German Confederation. Decisive controversy arose due to the passing of the November Constitution, which integrated the Duchy of Schleswig into the Danish kingdom in violation of the London Protocol.

Reasons for the war were the ethnic controversy in Schleswig and the co-existence of conflicting political systems within the Danish unitary state.