The Soviet Union invaded Finland, starting the Winter War

On November 30, 1939, three months into the Second World War, the Soviet Union launched a massive invasion of Finland, opening what became known as the Winter War. A staged border incident gave Moscow its pretext after Finland refused to cede territory near Leningrad.

Soviet planners expected a swift victory. Instead, Finland’s small, under-resourced army turned the frozen forests and lakes into a defensive maze, using ski troops, camouflage, and bitter cold to maul far larger Soviet columns. Resistance held for months despite minimal outside help.

A nation of fewer than four million held off one of the world’s great powers for over a hundred days.

By early February 1940 the Finnish lines were exhausted and being overrun. Finland signed the Treaty of Moscow on March 12, 1940, surrendering about 11 percent of its territory. The war humiliated the Soviet military and exposed weaknesses that other powers would soon note.