Franco - British plans for intervention in the Winter War During the early stages of World War II , the British and French Allies made a series of proposals to send troops against the Soviet Union , which had invaded Finland in 1939 as a consequence of the Molotov - Ribbentrop Pact . The plans involved the transit of British and French troops and equipment through neutral Norway and Sweden . The initial plans were abandoned due to Norway and Sweden declining transit through their land , fearing their countries would get pulled into the war . The Moscow Peace Treaty ended the war in March 1940 precluding the possibility of intervention . Background . In February 1940 , a major Soviet offensive broke through the Mannerheim Line on the Karelian Isthmus , exhausting Finnish defenses and forcing the country ' s government to accept peace negotiations on Soviet terms . At the news that Finland might be forced to cede its sovereignty to the USSR , public opinion in France and Britain , already favorable to Finland , swung in favor of military intervention . When rumors of an armistice reached governments in Paris and London , both decided to offer military support . Initial Allied approaches . The first intervention plan , approved on 4 – 5 February 1940 by the Allied High Command , consisted of 100 , 000 British and 35 , 000 French troops that were to disembark at the Norwegian port of Narvik and support Finland via Sweden while securing supply routes along the way . Plans were made to launch the operation on 20 March under the condition of a formal request for assistance from the Finnish government ( this was done to avoid German charges that the Franco - British forces constituted an invading army ) . On 2 March , transit rights were officially requested from the governments of Norway and Sweden . It was hoped that Allied intervention would eventually bring the two still neutral Nordic countries , Norway and Sweden , to the Allied side by strengthening their positions against Germany — although Hitler had by December declared to the Swedish government that Franco - British troops on Swedish soil would immediately provoke a German invasion . However , only a small fraction of the Franco - British troops were intended for Finland . Proposals to enter Finland directly , via the ice - free harbour of Petsamo , had been previously dismissed . There was speculation in some diplomatic quarters , encouraged by German sources , that the true objective of the operation was to occupy the Norwegian shipping harbour of Narvik and the vast mountainous areas of the north - Swedish iron ore fields , from which it was assumed that the Third Reich received a large share of its iron ore ( actually 30 % in 1938 ) , regarded as critical to war production . If the governments of France and Britain later broke their pledge not to seize territory or assets in Norway and Sweden , and Franco - British troops later moved to halt exports to Germany , the area could become a significant battleground between the Allies and the Germans . The Franco - British plan , as initially designed , proposed a defense of all of Scandinavia north of a line Stockholm – Gothenburg or Stockholm – Oslo , i . e . the British concept of " the Lake line " following the lakes of Mälaren , Hjälmaren , and Vänern , which would provide a good natural defense some 1 , 700 – 1 , 900 kilometres ( 1 , 000 - 1 , 200 miles ) south of Narvik . The expected frontier , the " Lake line , " not only involved Sweden ' s two largest cities , but could potentially result in large amounts of Swedish territory being either occupied by a foreign army or located in a potential war zone . Later , the plan was revised to include only the northern half of Sweden and the rather narrow adjacent Norwegian coast . Norwegian and Swedish reaction . Despite this compromise , the Norwegian government denied transit rights to the proposed Franco - British expedition . The Swedish government , headed by Prime Minister Per Albin Hansson , also declined to allow transit of armed troops through Swedish territory , in spite of the fact that Sweden had not declared itself neutral in the Winter War . Instead , the Swedish government argued that , since it had declared a policy of neutrality in the war between France , Britain , and Germany , the granting of transit rights by Sweden to a Franco - British corps , even though it would not be used against Germany , was still an illegal departure from international laws on neutrality . This strict interpretation appears to have been merely a pretext to avoid angering the Soviet and Nazi German governments , as it was abandoned after fifteen months . On 18 June 1941 , the Swedish government quickly agreed to German demands for transit rights across Sweden for German troops on their way from occupied Norway to Finland , in order to join the German attack on the Soviet Union . A total of 2 , 140 , 000 German soldiers and more than 100 , 000 German military railway carriages would cross neutral Swedish territory for the next three years . The Swedish Cabinet also decided to reject repeated Finnish pleas for regular Swedish troops to be deployed in Finland , and in the end the Swedes also made it clear that their present support in arms and munitions could not be maintained for much longer . Diplomatically , Finland was squeezed between Allied hopes for a prolonged war and Swedish and Norwegian fears that the Allies and Germans might soon be fighting each other on Swedish and Norwegian soil . Also , Norway and Sweden feared an influx of Finnish refugees if Finland lost to the Soviets . Further Allied proposals and their effect on peace negotiations . While Germany and Sweden pressured Finland to accept peace on unfavorable conditions , Britain and France had the opposite objective . Different plans and figures were presented for the Finns . At first , both France and Britain promised to send 20 , 000 men who were to arrive by the end of February . By the end of that month , Finland ' s Commander - in - Chief , Field Marshal Mannerheim , was pessimistic about the military situation . Therefore , on 29 February the government decided to start peace negotiations . That same day , the Soviets commenced an attack against Viipuri . When France and Britain realized that Finland was considering a peace treaty , they gave a new offer of 50 , 000 troops , if Finland asked for help before 12 March . Through Soviet agents in the French and British governments , indications of Franco - British plans reached Stalin , and may have contributed heavily to his decision to increase military pressure on the Finnish Army , while at the same time offering to negotiate an armistice .