Hitler threatened the USSR over a decade before he invaded. He wrote it in Mein Kampf. It was out in the open. Stalin attempted to get Western Europe to take the threat seriously, but Western European leaders understood that the primary target of the Third Reich was the USSR and they all wanted the Third Reich to win that war. Stalin never believed that the Third Reich would be an ally, and the attempt of people to spin it that way is so intellectually dishonest it boggles the mind.