When the king of Denmark died in late 1863, a controversy over Schleswig-Holstein gave Bismarck further opportunities. In brief, the duchies of Schleswig and Holstein at the southern base of the Danish peninsula had been ruled by the king of Denmark, but not as part of Denmark.
A fifteenth-century guarantee assured the duchies that they could never be separated from one another. Yet Holstein to the south was a member of the German Confederation; Schleswig to the north was not. Holstein was mostly German in population; Schleswig was mixed German and Danish.
