In the Austro–Prussian War of 1866, Prussia took Holstein from Austria and the two duchies subsequently merged into the Province of Schleswig-Holstein. From this point on, the Schleswig-Holstein Question was subsumed by the larger issue of Austro–Prussian relations, which the 1866 war deeply influenced. It survived, however, as between Danes and Germans, though narrowed to the question of the fate of the Danish population of Schleswig. This question is of great interest to students of international law and as illustrating the practical problems involved in asserting the modern principle of nationality.