Really? By that rationale, would New York City be a valid military target today?
Any reservations about the use of our Atomic Bombs is absurd.
No, it's not. It's based on sound moral principles, in fact.
As for NYC, possibly, but that is a typically ridiculous semantic.
Moral principles? Nonsense.
What was the ultimate difference between the results of Hiroshima being bombed and Tokyo being bombed?
Such as? Under the circumstances, the use of two atomic bombs was morally sound. The alternative (your alternative) is stalemate and a militarized Japan that remains a threat until they develop nuclear weapons. Then, if we are lucky, a Cold War in which we hope Japan does not ally with the Soviets.
Your other question is what are the limits to war? There are none and destruction of the world is possible. But it would not be the USA destroying the world.