The Soviets demanded that Finland cede it territory near the border, claiming that it was necessary for Soviet defense. The Fins refused, so on November 30th 1939 the Soviets invaded. The Soviets had expected a rapid victory. They had three times as many troops 30 times as many planes and 100 times as many tanks. However the purges of the Soviet military in the preceding decade had deprived the Soviets of many of their experienced officers and hurt moral, while the Finish Army was well led and highly motivated.

The major Finnish defense was a series of fortification called the Mannerheim Line. The Fins deployed some of their troops between the line and the Soviet border to harass and delay the advancing Soviet troops. By December 6th all Finnish troops had withdrawn to the line. The Soviets made repeated attempts to break through the Mannerheim line and were repulsed each time. In a series of battles with names such as the Battle of Suomussalmi and the Battle of Raate Road the Fins used superior tactics, better knowledge of the terrain and better ability to fight in the cold and snow to defeat much larger Soviet forces.

By the beginning of February the Soviets began to change their tactics while streaming every larger number of troops to face the exhausted Finnish Army. On February 15th the Finns ordered a partial withdrawal from the Mannerheim line to the next line of defense as the Soviets advanced. Meanwhile there was a renewed attempt at negotiations. The Finnish Army was exhausted and outnumbered, while the Soviets had been embarrassed and feared potential British or French intervention. On March 12, 1940 a formal peace agreement was signed in Moscow bringing the Winter War to an end. The Fins were forced to cede more territory to the Soviets than the Soviets had initially demanded before the war.