One thing historians consider very reliable about Stalin is that he would accuse others of doing things he did or was planning to do. There are events that historians pin on Stalin with no other evidence than the fact that he accused someone else of doing it.
One thing he accused FDR of doing was waiting out the war so that by the time Europe was exhausted, he could swoop in and conquer the winner easily and take over Europe.
Stalin was caught completely off guard, for two weeks immediately after the attack he was cowered in his room alone, until he was talked out of it, and finally re-assumed leadership.