Germany lacked
iron ore for
steel production and before the war, iron ore had been imported from mines in the
French region of
Lorraine. Since the outbreak of war in September 1939, that supply had been cut off and shipments from the other large supplier,
Sweden, were essential for the production of military equipment. In the
Gulf of Bothnia, the northern part of the
Baltic Sea, lies the Swedish port of
Luleå from where ore is shipped during the summer. The Baltic that far north froze in winter and for several months each year, the Swedes sent iron ore by rail to the
ice-free port of
Narvik, in the far north of Norway. In a normal year, 80 per cent 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 frozen. British intelligence suggested that Oxelösund could ship only a fifth of the weight that Germany required. Travelling inside Norwegian territorial waters for most of the trip, the shipping from Narvik was virtually immune to British interception. The British and the French
strategy was to use the
Winter War, the invasion by the
Soviet Union that has begun on 30 November 1939 as an excuse for seizing 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 there, they were to take control of Swedish harbours and mines, occupy cities such as
Gävle and Luleå, ending German access to Swedish ore and presenting 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, the Swedes and the Norwegians refused the transit requests. The Germans, aware of the danger, were making plans for an invasion of Norway to protect their supply of iron ore. 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 baulked the original Allied plan for using aid to Finland as a pretext for sending troops but on 12 March, the Allies decided to try a "semi-peaceable" invasion instead. Troops were to be landed in Norway and were to move into Sweden to capture the Swedish mines but if serious military resistance from the Norwegians was encountered, the Allies were not to press the issue. Finland sued for peace on 13 March and the revised version of the plan had to be abandoned. Faced with this, the Allied instead began work on Plan R 4. The Germans knew something of the Allied plans. Intercepted radio traffic showed that Allied transport groups had been readied. Later interceptions informed Germany that the Allies had abandoned the plan and redeployed their forces.
Adolf Hitler feared that the Allies would launch their invasion sooner or later and 9 April was set as the date of
Operation Weserübung, the German attack on
Denmark and Norway. == Plan ==