IMO, it would have saved millions of lives. Germany could not have staved off a concerted drive by French forces if they attacked aggressively. A French offensive would have also probably kept the USSR from joining in on the attack on Poland. Hitler would have had to agree to an armistice. A follow-up question...would Hitler have survived such a defeat or would internal uprisings unseat the Nazi Party?

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It is sometimes harder to fight my superiors than the French.
-Heinz Guderian