On November 26, 1939, the Soviets faked the Finnish shelling of the
Russian town of Mainila. In the aftermath of the shelling, they demanded
that the Finns apologize and withdraw their forces 25km from the
border. Denying responsibility, the Finns refused. Four days later,
450,000 Soviet troops crossed the border. They were met by the small
Finnish army which initially numbered only 180,000. The Finns were badly
outnumbered in all areas during the conflict with the Soviets also
possessing superiority in armor (6,541 to 30) and aircraft (3,800 to
130).

The Winter War proved a costly victory for the Soviets. In
the fighting, they lost approximately 126,875 dead or missing, 264,908
wounded, and 5,600 captured. In addition, they lost around 2,268 tanks
and armored cars. Casualties for the Finns numbered around 26,662 dead
and 39,886 wounded. The Soviet's poor performance in the Winter War led
Hitler to believe that Stalin's military could be quickly defeated if
attacked. He attempted to put this to the test when German forces
launched Operation Barbarossa in 1941. The Finns renewed their conflict
with the Soviets in June 1941, with their forces operating in
conjunction with, but not allied to, the Germans.