I always find it interesting the amount of people who think that, just because Stalin was a power mad despot who murdered millions, that for some reason he would try to conquer Europe if Hitler wasn't around 
Stalin wasn't mad, he might have been evil, but not mad. He wanted to Rule Russia, and keep Russia safe. Starting a War with all of Europe is something that does NOT fit with his personality.
As to some of the other responses, I also find the amount of dispute on Japan interesting. That the discussion of weather Pearl harbor would still happen is something hotly debated.
Stalin wasn't mad, he might have been evil, but not mad. He wanted to Rule Russia, and keep Russia safe. Starting a War with all of Europe is something that does NOT fit with his personality.
As to some of the other responses, I also find the amount of dispute on Japan interesting. That the discussion of weather Pearl harbor would still happen is something hotly debated.