France does not Fall 1940 -> What happens to Poland.

Stalin was cautious because he was paranoid.
Paranoia makes it difficult to have faith.

Stalin didn't trust his immediate subbordinates in the party. They were either conspiring with capitalists, conspiring with fascists or plotting to usurp his position.
Stalin didn't trust the Red Army. It was either inadequate as demonstrated by defeats in the Polish-Soviet war and its embarrassment in Finland, staffed with old imperialist/capitalists/trotskyites or plotting to usurp his position.
Stalin didn't trust the average Soviet citizen. They had little loyalty in the still young USSR. That went double for minorities.
Stalin didn't even trust the reports from his own intelligence agencies!

All in all, if you believe just about everyone is out to get you (with the extremely strange exception of Hitler) embarking on risky foreign wars isn't sensible.
 
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