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I disagree. Nuclear war is not defined as "humanity goes extinct."

"A nuclear war wouldn't be allowing us to have this conversation now."

Provide some evidence for this unsubstantiated claim. What creates a 100% chance of human extinction if nuclear weapons are used? How do you address the fact that the end of WWII objectively disproves your claim that nuclear weapon exchange would cause total extinction?

Oh, nothing, just your opinion and assumptions? Check.

If we assume that both USA and USSR launched their nuclear missiles back then at various places across earth the course of history would have changed significantly disabling us from having this conversation.

It is only logical.

How do you address the fact that the end of WWII objectively disproves your claim that nuclear weapon exchange would cause total extinction?

Different situation then. USSR was appeased and in bed with USA.