The Winter War (Finnish: Talvisota, Russian: Советско-финляндская война – official, Зимняя война- unofficial, Swedish: Vinterkriget) began when the Soviet Union ruled by dictator 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.
League of Nations Expulsion of the USSR, December 14, 1939
Having taken cognisance of the resolution adopted by the Assembly on December 14, 1939, regarding the appeal of the Finnish Government,
1. Associates itself with the condemnation by the Assembly of the action of the Union of Soviet Socialist Republics against the Finnish State, and
2. For the reasons set forth in the resolution of the Assembly, in virtue of Article 16, paragraph 4, of the Covenant, finds that, by its act, the Union of Soviet Socialist Republics has placed itself outside the League of Nations. It follows that the Union of Soviet Socialist Republics is no longer a Member of the League.
See also: Russo-Finnish War Maps