Plan R 4
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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. The northern part of the Baltic Sea, called the Gulf of Bothnia, had a principal Swedish port called 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, south of Stockholm, which was not obstructed by ice. However, British intelligence suggested that Oxelösund could ship only one fifth of the weight Germany required.
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 seemed to be vitally important.
The Allies devised a plan to use the Soviet Union 30 November 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.
Realizing the danger of Allied/German occupation and of the war being waged on their territory, both the Swedes and the Norwegians refused the transit requests.
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 16 February 1940, convinced Hitler that the Allies would not respect 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 12 March 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 12 March, 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. 9 April was set as the date of Operation Weserübung, the German attack on both Denmark and Norway.
The UK invasion plan 
The Allied invasion plan had two parts: Operation Wilfred, and Plan R 4.
In operation Wilfred, set to take place on 5 April (but delayed to 8 April), 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 becoming 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 8 April 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.
Combat operations 
Although "Plan R 4" could not be executed as planned, Allied troops were swiftly sent to Norway to fight alongside the Norwegians. Real success was only achieved against the Germans in the Narvik area, bringing them close to surrender. The Allied troops consisted of 24,500 British, Norwegian, French and Polish troops, in particular marine infantry, French Foreign Legionnaires, and Polish mountain troops. The German troops were composed of 2,000 mountain troops and 2,600 seamen from the sunk German invasion flotilla. On 17 April 1940, Hitler ordered the German troops to evacuate to Sweden to be interned. See the Allied campaign in Norway. However, the successful German campaign against France and the Low countries led to an Allied troop re-deployment. Allied troops were evacuated from Norway by 8 June 1940.
See also 
- Franco–British plans for intervention in the Winter War
- Foreign support in the Winter War
- British occupation of the Faroe Islands in World War II
- Iceland during World War II
- Norwegian Campaign
- Project Catherine
- Winter War#Franco-British plans for a Scandinavian theatre
- Ziemke, Earl F. (2000) . "Chapter 2, The German Decision to Invade Norway and Denmark". Command Decisions. United States Army Center of Military History. CMH Pub 70-7.