Germany did not have a sufficient domestic supply of iron ore, which is used in the production of steel. Before the war, large quantities of iron ore had been imported from mines in the French province of Lorraine. Since September 1939, that supply had no longer been available and so shipments from the other large supplier, Sweden, were essential for the production of tanks, guns, ships, railcars, trucks and other implements of war. During the ice-free summer months, iron ore would be shipped from Swedish port of Luleå, in the Gulf of Bothnia, opening into the Baltic Sea. During the winter, when access to the port was blocked by ice, Swedish exports were transported by rail to the ice-free port of Narvik in northern Norway. An average of 80 percent of Sweden's iron ore export moved through Narvik. The only alternative in winter was a long rail journey to ice-free port of Oxelösund, south of Stockholm, on the Baltic Sea. British intelligence suggested that Oxelösund could ship only one fifth of the required iron ore needed by Germany. Travel inside Norwegian territorial waters for most of the trip the shipping from Narvik was virtually immune to British attempts at interception. To the Allies, stopping the shipping and thus starving German industry were vitally important. The Allies devised a plan to use the Soviet Union's 30 November 1939attack 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. The plan was to get Norwegian and Swedish permission to send an expeditionary force to Finland across Sápmi, ostensibly to help the Finns. Once in place, they were to proceed to take control of Swedish harbours and mines, to occupy cities such as Gävle and Luleå and to shut down German access to Swedish ore. That would present Norway and Sweden with a fait accompli. Because of the danger of Allied or German occupation and of the war being waged on their territory, both the Swedes and the Norwegians refused the transit requests. Meanwhile, the Germans had realised the need to protect their strategic supply lines from Anglo-French interjection. The Altmark Incident of 16 February 1940 convinced Hitler that the Allies would not respect Norwegian neutrality, and he ordered for the plans for an invasion to be hastened. The Scandinavian reluctance to allow Allied military access halted the original Allied plan. On 12 March, the Allies revised the plan. Troops would land in Norway and proceed into Sweden to capture the mines. If military resistance was encountered, they would not to press the issue. However, Finland sued for peace on 13 March, and the plan was abandoned. The Germans were partly aware of the Allied planning. Intercepted radio traffic showed Allied transport groups had been readied. Later interceptions informed Germany that the Allies had abandoned the plan and redeployed their forces. However, Hitler feared the Allies would launch their invasion sooner or later. As a result, 9 April was set as the date of Operation Weserübung, the German attack on both Denmark and Norway.
The plan
Britain had two plans: Operation Wilfred and Plan R 4. Operation Wilfred, set to commence on 5 April, was a British naval operation that involved the mining of the channel between Norway and her offshore islands to prevent the transport of Swedish iron ore through neutral Norwegian waters to be used to sustain the German war effort. It was hoped that this would provoke a German reaction by the threat of or actual troop landings. Either would result in a British deployment of 18,000 troops to close the railroad to Sweden and to capture Trondheim and Bergen. Plan R 4 was a contingency and was to be launched only if Germany invaded Norway. Related orders stated that "it is not intended that any Forces shall be landed in Norway until the Germans have violated Norwegian Territory, or there is clear evidence that they intend to do so". On 8 April, started laying mines in Norwegian waters. Its departure was followed by troopships. In the meantime, Germany invaded Norway. Operation Wilfred became null and void, and Plan R 4 was no longer feasible. British, French and Polish troops were then deployed to Norway to fight alongside the Norwegian military during the Norwegian campaign.
