I believe that Moscow would have fallen if Stalin had left. How demoralizing (spelling check) would it have been for the Moscow population if their leader just fled. It would of given the Russian people the impression that the city wasn't worth fighting for. Because if Stalin had left it would have fuelled the people to believe that there was no hope.

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And when he gets to heaven
To Saint Peter he will tell
One more soldier reporting sir
I have served my time in hell