Neutral Norway attracted both Allies and Germans. The northern port of Narvik was the only all-weather outlet for Swedish iron ore, important to Germany, and deep-water channels along the coast, known as the Leads, formed a valuable link between Germany and the North Atlantic. The Allies contemplated seizing Narvik, pushing on to the ore-fields and then aiding the Finns, but resolved to mine the Leads instead. No sooner had they begun, on April 8, than it became clear that a major German invasion was under way: neither fierce Norwegian resistance nor a series of botched Allied countermoves could prevent German occupation, completed by early June.
On February 16, 1940, Graf Spee's homeward-bound supply ship Altmark was boarded by men from HMS Cossack in Jössing Fjord, in Norwegian territorial waters. Almost 300 prisoners were freed, but the incident strained Anglo-Norwegian relations and drew Hitler's attention to the area.