The Soviet Union signed a non-aggression pact Germany and participated in the invasion of Poland. They also supplied Germany with aid during Germany’s war against Britain up until the day they were invaded. Winston Churchill hated Communism. However when Russia was attacked by Germany in WWII there was never any doubt who the allies were going to support. Churchill said “If Hitler invaded Hell I would make at least a favorable reference to the devil in the House of Commons”.
(Book – The Second World War Volume III – Winston S Churchill)