In 1863, nevertheless, the Liberal government prevailed on the new Danish king, Christian IX, to sign a new joint constitution for Denmark and Schleswig. Prussia and Austria were thus freed to intervene as the upholders of the 1852 protocol. In the ensuing Danish War (1864), Danish military resistance was crushed by Prussia and Austria in two brief campaigns, and, by the Treaty of Vienna in October, Christian IX ceded Schleswig and Holstein to Prussia and Austria.

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When the government in Copenhagen sought to make Schleswig an integral part of the Danish state in 1863, nationalist sentiment in central Europe was outraged. William I proposed to Francis Joseph that the two leading powers of the German Confederation should occupy the duchies in order to prevent the violation of an international agreement that had guaranteed their separate status. Afraid to let the Prussians act on their own, the emperor agreed, and in 1864 there was a brief war against Denmark that demonstrated the strength of the reorganized army of the Hohenzollerns. Danish hopes for foreign assistance proved illusory, and by the Peace of Vienna (October 30) the duchies became the joint possession of Prussia and Austria.