The German Invasion and Capture of Norway. Events Prior to the Invasion. World War II began when the Germans invaded and captured Poland in September 1939. Within a few days, the Germans unleashed their submarines against Allied shipping and the Battle of the Atlantic was on. In October 1939 the British mined the Strait of Dover and the Germans were forced to send their ships and submarines through the North Sea and Norwegian Sea around the north of Scotland to reach the Atlantic.
"Soviet Threat to the Atlantic Sea Lines of Communications: Lessons Learned from the German Capture of Norway in 1940,"
Naval War College Review: Vol. 34
, Article 5.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol34/iss3/5