Prussia and Austria wage war against Denmark

Since the Middle Ages, the duchies of Schleswig and Holstein had been considered “eternally undivided”, i.e. to belong together. Although governed separately, both territories were subject to the authority of the Danish king. In spite of this personal union, Holstein was part of the German Confederation, but Schleswig was not. The loose nature of the Confederation meant that this arrangement initially caused few problems, but the situation was soon complicated in the 1850s by growing calls for unification of the German-speaking lands.

As no agreement could be reached as how to resolve the “Schleswig Holstein question”, Prussia and Austria decided to do so by force of arms. Units of the Prussian and Austrian armies crossed the border river Eider into the Kingdom of Denmark on 1 February 1864. After they had succeeded in occupying the entire Jutland peninsula, Denmark agreed to relinquish its claims over the two duchies.