The Battle of Suomussalmi (7 December, 1939 - 8 January, 1940) was the decisive battle of the Winter War between Finland and the Soviet Union. Despite being heavily outnumbered in men and material, the Finns were able to stop the advance into their country by using their mobility and exploiting the Russians reliance on the few roads in the heavily forested terrain. The Finnish Army was also much better equipped to fight in the grueling winter conditions, while Russian equipment routinely froze in the cold, as did their under clothed soldiers.
The Soviet failure to successfully invade Finland heavily influenced Hitler’s decision to go ahead with Operation BARBAROSSA.