Second war of Schleswig
Second war of Schleswig also known as Danish war or Danish-Prussian war in 1864 was fought between Germany and Denmark.
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. Wilhelm I proposed to Francis Joseph that the two leading powers of the German Confederation, Prussia and Austria, should occupy the duchies of Schleswig and Holstein 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.
See also
da:2. Slesvigske Krig de:Deutsch-Dänischer Krieg sv:Dansk-tyska kriget 1864