Why was the Soviet Union expelled from the League of Nations in 1939?

The Soviet Union was expelled from the League of Nations in December 1939 due to its unprovoked military invasion of Finland on November 30, 1939, an act that initiated the Russo-Finnish War.

Following the defeat of Poland earlier that year, the Soviet Union demanded that Finland cede territory on the Karelian Isthmus, lease the Hanko Peninsula for a naval base, and surrender islands in the Gulf of Finland to improve the security of Leningrad. When Finland rejected these demands, the Soviet Union launched its offensive. The League of Nations condemned this aggression as a violation of international norms, leading to the expulsion of the Soviet Union from the organization.