Winter War: The Battle of Tolvajärvi, also known as the first major Finnish victory in the Winter War, begins.

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 Battle of Tolvajärvi was fought on 12 December 1939 between Finland and the Soviet Union. It was the first large offensive victory for the Finns in the Winter War.

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 has a population of 5.6 million, the majority being ethnic Finns. Its capital and largest city is Helsinki. The official languages are Finnish and Swedish, the mother tongues of 84.1 percent and 5.1 percent of the population, respectively. Finland's climate varies from humid continental in the south to boreal in the north. Its land is predominantly covered by boreal forest, with over 180,000 recorded lakes.

December 12 is the 346th day of the year in the Gregorian calendar; 19 days remain until the end of the year.