January 16, 1864
The Duchy of Holstein was a part of the German Confederation, while Schleswig was not. Denmark and Germany were fighting for control over these two duchies. Although the London Protocol of 1852 had tried to resolve the status of the duchies in relation to Denmark, there were continuous tensions between Denmark and Germany. In 1863, Denmark tried to integrate Schleswig more closely, violating the agreements in the London Protocol. Prussia and Austria, both seeking influence in the German Confederation and beyond, responded to this action by giving an ultimatum. Denmark rejected the ultimatum, leading to the invasion of Schleswig by Prussian and Austrian forces in early 1864.