In April 1940 Nazi Germany invaded Norway and, after a brief period of resistance, began an occupation that lasted until the end of the war. Norway subsequently became the most heavily fortified Axis-occupied country during the war with a ratio of one German soldier for every eight Norwegian citizens.
Civil rule by a pro-German collaborationist government led by Vidkun Quisling was effectively dictated by the Reichskommissariat Norwegen (Reich Commissariat of Norway), while the Norwegian King Haakon VII and his prewar government escaped to London where they acted as a government in exile.
As depicted in this trailer for the excellent TV series The Heavy Water War, Norwegian saboteurs carried out several operations to prevent Nazi Germany from acquiring heavy water (deuterium oxide) which was needed to produce nuclear weapons.