IF NYC was destroyed by a Nuke, would we really start flinging Nukes back, knowing that it might mean the end of this Planet as we know it?? The whole idea of having a Nuclear arsenal is that no one wants mutual destruction. If North Korea blew up NYC, we might go to war with them but using NUKES would bring China into the war, and they would use NUKES too.
It would be just as bad for both sides, and everyone including, other countries near by, and if they were all launched, we would be causing suicide, because no one would survive on this planet, once Nuclear Winter sets in.