This points to a flaw in your question.

You probably should have said “foolproof countermeasure” if you really just wanted to remove nukes as a factor to see what happens.

But you said “foolproof deterrent” and now you’re quibbling at people over whether a psychological deterrent can actually be foolproof.

Maybe not, but even your question is nonsensical. The fact is that we are already using guaranteed total destruction of the world as a deterrent and it has so far worked. What more deterrence are you even suggesting we might add to that???