The Winter War began when the Soviet Union led by Josef Stalin attacked Finland on November 30, 1939, three months after the invasion of Poland by Germany that started World War II.

Because the attack was judged as illegal, the Soviet Union was expelled from the League of Nations on December 14. Finnish resistance frustrated the Soviet forces, who outnumbered the Finns 4 to 1 in men, 200 to 1 in tanks and 30 to 1 in aircraft. The Soviet Union had just recently seen a drastic purge in 1937 that had crippled the Red Army, reducing its morale and efficiency just before the German invasion.

With up to 50% of army officers executed, including the vast majority of the highest ranking officers, the result was that the Red Army officer corps in 1939 had many inexperienced senior officers. Finland held out until their ammunition ran out and on February 29, 1940 they agreed to start negotations. When the Moscow Peace Treaty was signed ceding about 9% of Finland’s territory (excluding its population) and 20% of its industrial capacity to the Soviet Union.