What was the Schleswig-Holstein question?
It concerned whether or not Schleswig, the southern-most of the two dutchies, with a German majority, would stay with Denmark as Schleswig-Holstein. The argument went on from 1848 to 1864, included two wars between the German Confederation and Denmark, and much civil unrest. Finally, in 1864 it was settled with Prussian troops routing the Danes. Schleswig was not returned to Denmark until after WWI.