In 1864, Denmark ceded Schleswig (which the Danes call Slesvig) and
Holstein to Prussia and Austria (both came in 1866 to Prussia, and were
included in the unification of 1871).

The allocation of Holstein to Germany was never questioned by the Danes,
Schleswig was contested. Originally - in the middle ages - Danish land,
during the 16th to 18th century Schleswig and Holstein had a common history,
separate from and often in conflict with Denmark. The common parliament of
Schleswig-Holstein chose (lower) German as their official language, and a
considerable segment of the Schleswig population developed, over the
centuries, a Schleswig-Holsteinish / German identity, while a considerable
part of the peasantry, especially in the northern section of Schleswig, felt
Danish.

Thus, the Schleswigers in the 19th century, were split in a camp with a
German, and a camp with a Danish identity (actually there is a third,
Northern Frisian camp). The treaty of 1864 foresaw the questioning of the
population of Northern Schleswig by plebiscite (if they wanted their land to
be part of Germany or Denmark); the Germans failed to hold that plebiscite.

After Germany lost WW I (in which Denmark took no part) that plebiscite
was enforced, only in parts of Schleswig. Two thirds of Schleswig remained
German, with the city which gave the area her name; the northern section was
separated and united with Denmark. The area remaining with Germany is called
Sønderjylland by the Danes; the German Bundesland Schleswig-Holstein has a
Danish minority, which is guaranteed a seat in the state legislature.