Let us begin in 1917.
- Of course, 1917 was one of the most important years in the history of Russia. It was in this year that the Tsar was overthrown and the Russian Civil War started.
- In 1918, the Soviet Union pulled out of World War I. Many small countries that had been part of the Russian Empire broke away from Russia at that time.
- In 1924, Vladimir Lenin dies and is replaced by Joseph Stalin as leader of the Soviet Union. Stalin will rule until 1953. His death in 1953 will be the last of the important dates in the time period you are asking about.
- In 1939, the Soviet Union and Nazi German sign a non-aggression pact.
- In 1941, the Nazis break this pact and invade the Soviet Union, thus really starting Soviet involvement in WWII.
- In 1945, the Soviet Army reaches Berlin and WWII ends for the Soviets (to all intents and purposes).