I have no idea who knew what or when, but the facts stand that 1) the war started when Germany & the Soviet Union invaded Poland, but war was only declared on Germany by the “Western Allies”, and 2) while Americans died fighting the Japanese in the Pacific, the Soviet Union had peace with them. In the end, we saved the USSR so they could have Poland (and Latvia, Lithuania, Estonia, Czechoslovakia, Hungary, Romania, Bulgaria...)
What a hoax.
It was certainly the case that the U.S. could not cure all the evils in the world in fighting the Nazis with the Soviet Union as an ally.
What do you think the world would have looked like for the last seventy years if Hitler had not attacked the Soviet Union? Eventually Hitler would have taken on the Soviet Union, but he might have consolidated his gains in Europe and subjugated the British first.