The Winter War also known as First Soviet-Finnish 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 the organisation.
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 from the Finnish border. When Finland refused, the USSR invaded. Most sources conclude that the Soviet Union had intended to conquer all of 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.