The Non-Aggression Pact and war
On 23 August 1939, Stalin and Hitler sign a non-aggression pact, also known as the Molotov-Ribbentrop pact. The pact makes it possible for Stalin to gain control of new areas west of the Soviet Union. Estonia, Latvia, Lithuania and Bessarabia are included in the Soviet Union. In addition, Germany and Soviet agree to divide Poland between them.
On 1 September, Germany invades Poland. Great Britain and France respond by declaring war on Germany. Soviet troops occupy eastern Poland.