Ok, but then what is the purpose of revenge? It is completely expected that citizens and perhaps the individual leaders of the besieged country wants to exact the same kind of treatment it was dealt, but as a military establishment deciding the country's future, what is the end benefit of destroying your attacker?
There is only one scenario in which case your logic would apply: when Country A attacks a portion of Country B. In that case, Country B can launch a retaliatory attack to deter further strikes on its territory. But in a scenario in which Country B is completely destroyed, is there a logical necessity to retaliate?
Of course there is -- in a scenario where Country B is completely destroyed, nuclear retaliation becomes even more pressing so as to inflict as significant of casualties on Country A who had caused genocide against you in the first place.
If your country no longer exists and you have the option to exact revenge, then if you are a military commander then retaliation becomes not merely an option but a duty.
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Frankly, I'm not going to reply further to this particular thought experiment, but I want to make clear in my moderator shoes, that you should tread significantly more carefully in the future.
I am not sure if you posted this "thought experiment" simply arising from being naive and not thinking it through, or if it is a case of being deliberately and quietly malicious.
Either way, this seemingly innocent "thought experiment" -- when read with the immediately preceding debate about Chinese nuclear weapon requirements -- will inevitably be interpreted and read by some users as not-so-subtly imagining a nuclear scenario between China and the US, and suggesting that one of these countries, if suffering a nuclear attack by the other, should consider it appropriate to not retaliate.
I say "one of these countries," but with the phrasing of your original post and the current balance of nuclear power in the real world means people will reasonably imagine a scenario whereby China suffers a nuclear attack by the US and should not launch nuclear retaliation following an attack.
Either intentional or not -- and frankly I've read enough of your posts to harbour doubt -- the mere suggestion of China (or the US for that matter) suffering a nuclear attack and then floating the idea that they should not retaliate, is ludicrous and massively inflammatory.
It is the equivalent of "innocently" asking -- if China (or country XYZ) suffers a nuclear genocide from an opposing country, what real benefit is there in retaliating against the country that launched it?
The only reason I am not banning you for a lengthy duration as a warning shot is because of your long history here and because of plausible deniability that you "didn't know what you were doing". But honestly, it is rather close.
Therefore consider this as a mere formal warning to consider the implications of what you're writing more carefully in future, so as to avoid unintentionally stepping on people's toes.