In November 1939, after its annexation of eastern Poland, the Soviet Union used a staged incident to demand territorial concessions from Finland. The Finns refused, and the Soviet army invaded, expecting an easy victory. The Finnish army was forced back at first, but then held the line and inflicted heavy casualties on the Soviet forces. While heavily out-numbered, the Finns were better suited for winter combat than the Russians, and used guerilla tactics, including use of the Molotov Cocktail, to great advantage. In addition, Stalin's purges had weakened the Soviet army. In February, the Soviets launched a new offensive, and the Finns were forced to sue for peace the next month, ceding the territories that the Soviet Union demanded. 
Finland lost 25,000 killed in the war, while the USSR lost 127,000 killed or missing.
- Battle, by R.G. Grant, DK Publishing, 2005
- Finnish Aces of World War 2, by Kari Stenman, Osprey Publishing, 1998