The Winter War was a military conflict 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. The League of Nations deemed the attack illegal and expelled the Soviet Union from the organisation.
The conflict began after the Soviets sought to obtain some Finnish territory, demanding among other concessions that Finland cede substantial border territories in exchange for land elsewhere, claiming security reasons—primarily the protection of Leningrad, 32 km from the Finnish border. Finland refused, and the USSR invaded the country. Many sources conclude that the Soviet Union had intended to conquer all Finland, and use 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 the full Soviet conquest. Finland repelled Soviet attacks for more than two months and inflicted substantial losses on the invaders while temperatures ranged as low as −43 °C.