I read a book and they summed up the thing very well. Whoever has the most men and resources to fall back on is going to win. In any protracted “total war” if you have a piece of territory that extends from Bielorussia all the way to vladivostok you would need hundreds of millions of troops to fight all the way there (in era before nuke bomb). If Hitler had concentrated on invading the UK and promised Stalin the island of Ireland for keeping out of trouble I am pretty sure that would have worked. A free Atlantic base with low native population for the USSR (who would have left en mass) and the Wehrmacht could have finishedthe job with the UK that is nothing more than a lightweight pumped up with american steroids. London – Coventry would have been obliterated within a month.