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alternatehistory.com
The 1939 Soviet invasion of Poland was a Soviet military operation that started without a formal declaration of war on 17 September 1939, immediately after the undeclared war between the Soviet Union and the Empire of Japan.
Lets move up the time table a bit and the undeclared war between Japan and the USSR ends earlier and Stalin decides to invade Poland on the same day as Hitler does. France and the UK decide to declare war on both of them.
After Poland and other parts of Eastern Europe are taken by the Red Army Stalin decides to take the Middle East from the UK while Germany is busy with France and he invades Iraq and Iran.