or Winter War
(1939–40) War waged by the Soviet Union against Finland at the start of World War II , following the signing of the German-Soviet Nonaggression Pact .
When Finland refused to grant the Soviets a naval base and other concessions, Soviet troops attacked on several fronts in November 1939. The heavily outnumbered Finns under Carl Gustav Emil Mannerheim put up a skillful defense until February 1940, when heavy Russian bombardments breached the Finns' southern defenses. A peace treaty in March 1940 ceded western Karelia to Russia and allowed construction of a Soviet naval base on the Hanko peninsula.