BB, Hitler started WWII. Stalin merely reacted later and joined the war to tan Nazi hide.
Seanus, Hitler and Stalin started WWII. For western Europeans it was Hitler, ask the Baltic states who started WWII from their point of view.
They both had their own agenda, and knew that in the end they will confront but each wanted to dominate first his part of Europe. Nazi Germany waged war with the West while Soviet Russia was occupying the Baltic States. Ask Finland, Latvia, Lithuania and most of all ask Poland.
You know, Europe is not only Western Europe, it's the whole bloody continent, and Eastern Europe has a history which for so long you didn't want to hear about it.
In the end Stalin went on the side of the Allies, but this only because Hitler was too much trouble for him. You know, when there was this Anticomintern pact, Hitler and Stalin were in such good relations that a joke circulated around that Stalin was about to join the Anticomintern side.