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.

Finland, officially the Republic of Finland, is a Nordic country in Northern Europe. It borders Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bothnia to the west and the Gulf of Finland to the south, opposite Estonia. Finland covers an area of 338,145 square kilometres (130,559 sq mi) and has a population of 5.6 million. Helsinki is the capital and largest city. The vast majority of the population are ethnic Finns. The official languages are Finnish and Swedish, of which 84.9 percent and 5.1 percent of the population speak the first as their mother tongue. Finland's climate varies from humid continental in the south to boreal in the north. The land cover is predominantly boreal forest biome, with more than 180,000 recorded lakes.