This is pretty deep into hypothesis than reality on ground.This is pretty nonsensical, hitting American command and control/ major population centers with nuclear weapons, regardless of yield, is an invitation to a massive retaliation. Not to mention US tactical SLBM capability. Once the nuclear step is taken, Shanghai and all the other cities that hold the manufacturing base start looking like a pretty target with all those fancy lights at night, yaknow?
A nuclear power will go nuclear knowing there is going to be retaliation. So taking out second strike capability is not part of equation. When this scenario becomes reality, it means one side has decided time is up for the other and certain line has been crossed.
I have mentioned before, in this scenario, this is not simply a button push. The entire nation will have to make a life/death decision.
Now, in case of China Russia or even India, Pakistan, this decision will be taken by their citizens. Who will do for USA?
USA military capability is part of the cycle that keeps the people of USA happy by keeping their life style and demand afloat. It is not a military in true sense.
If China nukes USA, USA will have to decide, either it can live with whatever of it is left. Or MAD.
But why would China nuke USA? USA as it is now gives China lots of money, so it is more useful to China alive than dead.
Similarly i have been reading the posts above, i disagree with all of these notions/policy of USA's launch on warning,launch on watch, launch on breakfast bullshit.
USA can not fight a nuclear war. Nuclear weapons are insurance of USA's existence like Israel. And it only exists as long as the opposite side does not find it attractive to choose death for both side.
But there are nations that will go to such length, which is why, USA and Israel has no choice but to keep nuclear materials out of those nation's reach.