Danish nationalists believe that Schleswig, but not Holstein, should be a part of Denmark, as Schleswig contains a large number of Danes, while Holstein does not.

German nationalists believe that Schleswig, Holstein, and Lauenburg should remain united, and their belief that Schleswig and Holstein should not be separated has led to the two duchies being referred to as Schleswig-Holstein.

Schleswig has become a particular source of contention, as it contains a large number of Danes, Germans and North Frisians.

Another cause of the war is the legally questionable change to the rules of ducal succession in the duchies.