Note that while this wasn't widely known in the West at the time and therefore cannot be their justification for going to war against Hitler, Nazi Germany's "Generalplan Ost" [0] (their "General Plan for the East", a plan of global genocide so vast it defies belief) called for the almost total extermination of the populations of most Slavic countries, Russia and Poland included. If I remember correctly, the quota for Poland was 80% Poles -- and note I say Poles, not Jewish Poles -- to be exterminated.
So on the one hand, Cold War Poland ended up a puppet of the Soviets. Repression of dissidents, etc. But the Poles were alive. On the other hand, had Nazi Germany won and completed their plan there would be no Poles left to complain about their lack of freedom.
Except I doubt it would ever have happened. Germany would have struggled mightily with Russia. Likely Germany and Russia would have fought each other to some sort of standstill. They would have bled each other out and which point allied forces could have moved in and beaten both Germany and Russia. We could have avoided the whole cold war and east block.
So on the one hand, Cold War Poland ended up a puppet of the Soviets. Repression of dissidents, etc. But the Poles were alive. On the other hand, had Nazi Germany won and completed their plan there would be no Poles left to complain about their lack of freedom.
So was WWII really a mistake?
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[0] https://en.wikipedia.org/wiki/Generalplan_Ost