Finnish Democratic Republic in the context of Finnish Karelia

The Winter War was a war between the Soviet Union and Finland. It began with a Soviet invasion of Finland on 30 November 1939, three months after the outbreak of World War II, and ended three and a half months later with the Moscow Peace Treaty on 13 March 1940. Despite superior military strength, especially in tanks and aircraft, the Soviet Union suffered severe losses and initially made little headway. The League of Nations deemed the attack illegal and expelled the Soviet Union from its organization.

The Soviets made several demands, including that Finland cede substantial border territories in exchange for land elsewhere, claiming security reasons — primarily the protection of Leningrad, 32 km (20 mi) from the Finnish border. When Finland refused, the Soviets invaded. Most sources conclude that the Soviet Union had intended to conquer all of Finland, and cite the establishment of the puppet Finnish Communist government and the Molotov–Ribbentrop Pact's secret protocols as evidence of this, while other sources argue against the idea of a full Soviet conquest. Finland repelled Soviet attacks for more than two months and inflicted substantial losses on the invaders in temperatures as low as −43 °C (−45 °F). The battles focused mainly on Taipale along the Karelian Isthmus, on Kollaa in Ladoga Karelia and on Raate Road in Kainuu, but there were also battles in Lapland and North Karelia.