Plan R 4

Plan R 4 was the World War II British plan for an invasion of the neutral states of Norway and Sweden in April 1940. Earlier, the British had planned a similar intervention with France during the Winter War.

Germany did not have a sufficient domestic supply of iron ore, used in the production of steel. Before the war, large quantities of iron ore were imported from mines in the French province of Lorraine. Since September 1939, this supply was no longer available. So shipments from the other large supplier, Sweden, were essential for the production of tanks, guns, ships, rail cars, trucks and other implements of war. In the northern part of the Baltic Sea, the Gulf of Bothnia, lies the Swedish port of Luleå from where in the summer a quantity of ore was shipped. It was frozen in winter, so for several months each year the Swedes shipped most of their iron ore by rail through the ice-free port of Narvik, in the far north of Norway. In a normal year, 80% of the iron ore was exported through Narvik. The only alternative in winter was a long rail journey to Oxelösund on the Baltic Sea, south of , which was not obstructed by ice. However, British intelligence suggested that Oxelösund could ship only one fifth of the weight Germany required.

Travelling inside Norwegian territorial waters for most of the trip the shipping from Narvik was virtually immune to British interception attempts. To the Allies, stopping the shipping and thus starving German industry was vitally important.

The Allies devised a plan to use the Soviet Union's 30 November 1939 attack on Finland as a cover for seizing both the Swedish ore fields in the north, and the Norwegian harbours through which it was shipped to Germany.

...
Wikipedia