What incident triggered the invasion of Norway?

Short answer

The Altmark incident: in February the British destroyer HMS Cossack boarded the German tanker Altmark in Norwegian waters. Hitler saw it as clear proof that the Allies would also violate Norwegian neutrality and hardened his resolve to invade; on 19 February 1940 he ordered planning to be intensified.

Published: November 1, 2026 · Updated: November 1, 2026 · Edited by Above Ten Thousand · Editorial policy

What incident triggered the invasion of Norway?

Altmark Incident

The boarding of the Altmark

In February, the Royal Navy destroyer HMS Cossack boarded the German tanker Altmark in Norwegian waters. Hitler saw the incident as a clear sign that the Allies were also prepared to violate Norwegian neutrality, which made him commit even more firmly to the invasion of Norway. On 19 February 1940 orders were given to intensify planning for attacks on Norway and Denmark, which eventually took place on 9 April 1940 under the code name Operation Weserübung.

Before and after the incident

After a meeting with the Norwegian Vidkun Quisling on 14 December, Hitler turned his attention to Scandinavia. On 21 February 1940 command of the operation was handed to General Nikolaus von Falkenhorst. The capture of fighter bases and sites for air-warning stations was added to the plan at the Luftwaffe's insistence.

The invasion

In the early hours of 9 April 1940 — Wesertag, "Weser Day" — German forces occupied Denmark and invaded Norway. The operation ran from 9 April to 10 June 1940 and was the opening action of the Norwegian campaign.