Winter War Germany's iron-ore supplies Karelian Isthmus Viipuri Mannerheim Line Molotov–Ribbentrop Pact peace treaty Soviet Union German
The August 1939 Molotov–Ribbentrop Pact between Germany and the Soviet Union contained a secret protocol dividing much of eastern Europe and assigning Finland to the Soviet sphere of influence. After unsuccessfully attempting to force territorial and other concessions on the Finns, the Soviet Union tried to invade Finland in November 1939 during the Winter War, intending to establish a communist puppet government in Finland. The conflict threatened Germany's iron-ore supplies and offered the prospect of Allied interference in the region. Despite Finnish resistance, a peace treaty was signed in March 1940, wherein Finland ceded some key territory to the Soviet Union, including the Karelian Isthmus, containing Finland's second-largest city, Viipuri, and the critical defensive structure of the Mannerheim Line. After this war, Finland sought protection and support from the United Kingdom and non-aligned Sweden, but was thwarted by Soviet and German actions. This resulted in Finland being drawn closer to Germany, first with the intent of enlisting German support as a counterweight to thwart continuing Soviet pressure, and later to help regain lost territories.
