The Winter War was a military conflict between the Soviet Union (USSR) and Finland lasting three and a half months from 1939 to 1940. The war began with the Soviet invasion of Finland on 30 November 1939, which was three months after the outbreak of World War II, and ended with the Moscow Peace Treaty on 13 March 1940. The League of Nations deemed the attack illegal and expelled the Soviet Union from the League.
The Soviet Union sought to obtain parts of 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 (20 mi) 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 in temperatures down to −43 °C (−45 °F). After reorganisation and adoption of different tactics, the Soviets renewed their offensive and overcame Finnish defenses.
Hostilities ceased in March 1940 with the signing of the Moscow Peace Treaty. Finland ceded territory representing 11 percent of its land area and 30 percent of its economy to the Soviet Union. Soviet losses were heavy, and the country's international reputation suffered. Soviet gains exceeded their pre-war demands and the USSR received substantial territory along Lake Ladoga and in Northern Finland. Finland retained its sovereignty and enhanced its international reputation. The poor performance of the Red Army encouraged Adolf Hitler to think that an attack on the Soviet Union would be successful and confirmed negative Western opinions of the Soviet military. After 15 months of Interim Peace, in June 1941, Nazi Germany commenced Operation Barbarossa and the Continuation War between Finland and the USSR began.