Traveling 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 November 30, 1939, attack on Finland as a cover for seizing both the important Swedish ore fields in the north, and the Norwegian harbors through which it was shipped to Germany.
The plan was to get Norwegian and Swedish permission to send an expeditionary force to Finland across northern Norway and Sweden, ostensibly to help the Finns. Once in place they were however to proceed to take control of the harbors and mines, occupying cities such as Gävle and Luleå and shutting down the German access to Swedish ore, presenting Norway and Sweden with a fait accompli.
Meanwhile, the Germans having realized the Allied threat, were making plans for a possible pre-emptive invasion of Norway in order to protect their strategic supply lines. The Altmark Incident of February 16, 1940, convinced Hitler that the Allies would not respect the Norwegian neutrality, and he ordered the plans for an invasion hastened.
The Scandinavian reluctance to allow Allied troops on their territory halted the original Allied plan for using aid to Finland as a pretext for moving in troops, but on March 12 the Allies decided to try a "semi-peaceful" invasion nevertheless. Troops were to be landed in Norway, and proceed into Sweden to capture the Swedish mines. However, if serious military resistance was encountered they were not to press the issue. However, Finland sued for peace on March 12, so the revised version of this plan had to be abandoned too.
The Germans were partly aware of the Allied planning, they intercepted radio traffic showing that Allied transport groups were being readied, and a few days later messages that the Allies had had to abandon their plan and redeploy their forces.
Plans for the German invasion of Norway continued since Hitler feared the Allies were nevertheless going to launch their own invasion sooner or later, and he was right although he was unaware of the actual plans. April 9 was set as the date of Operation Weserübung, the German attack on Norway.
In operation Wilfred, to take place on April 5 (but delayed to April 8), the Norwegian territorial waters were to be mined, violating Norwegian neutrality. This would force the ships carrying ore to Germany to travel outside the protection of Norwegian territorial waters and thus accessible to the British navy.
It was hoped that this would provoke a German military reaction. As soon as the Germans would react, either by landing troops in Norway or demonstrating the intention to do so, a British force would be landed in Norway, 18,000 Allied troops were to land in Narvik, closing the railroad to Sweden. Other cities to be captured were Trondheim and Bergen.
The first ship with Allied troops were to start the journey a few hours after the mine laying. On April 8 a Royal Navy detachment led by HMS Renown mined Norwegian waters in operation Wilfred, but German troops were already on their way, and the original "Plan R 4" was no longer feasible. The Allies had however provided Hitler with an invasion excuse.