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Operation Weserübung (German: [ˈveːsɐˌʔyːbʊŋ]) was the code name for Germany's assault on Denmark and Norway during the Second World War and the opening operation of the Norwegian Campaign. The name comes from the German for Operation Weser-Exercise (Unternehmen Weserübung), the Weser being a German river.

In the early morning of 9 April 1940 (Wesertag; "Weser Day"), Germany invaded Denmark and Norway, ostensibly as a preventive manoeuvre against a planned, and openly discussed, Franco-British occupation of Norway. After the invasions, envoys of the Germans informed the governments of Denmark and Norway that the Wehrmacht had come to protect the countries' neutrality against Franco-British aggression. Significant differences in geography, location and climate between the two countries made the actual military operations very dissimilar.
https://en.wikipedia.org/wiki/Operation_Weserübung