5/ Denmark's other land border is with
Germany, and was set via plebiscite in 1920 thus finally resolving "the Schleswig-Holstein question", a repeated source of conflict in 19th century
https://en.wikipedia.org/wiki/Schleswig%E2%80%93Holstein_question
Today there is an officially recognised German-speaking minority in Denmark, and an equivalent Danish-speaking minority in Germany:
https://de.wikipedia.org/wiki/Deutsche_Minderheit_in_D%C3%A4nemark
https://en.wikipedia.org/wiki/Danish_minority_of_Southern_Schleswig