I would guess there was alot of behind the scenes talks between Finland and Germany in the late 30's. The Germans and Finns would have had agents in Russia and could estimate the poor condition of the Russian armed forces and the Germans would have been planning for it's attack of Russia in the late 30's. The poor Finns had to choose who was the bigger threat, Hitler or Stalin.
I suppose Stalin could have attacked Finland after the end of WWII and no one would have done anything to help but for some reason he choose not to. Perhaps he was happy with all the land he gained in eastern Europe.
