Imagine Haiti taking place in a major US city. That would be from ONE NUCLEAR WEAPON.
Now imagine twenty cities, fifty, etc.
It's even worse then that.
There's a little known book called (my dodgy memory says) Aftermath, in which 2 Brittish cities get accidentally nuked (one was London, the other, I think Manchester)
The described damage made Haiti look like a walk in the park. In the book, the Brittish economy basically crashed instantly.
Isn't US population even more centralised then that of Brittain?
I'd imagine, a douzen nukes would wipe out the US as any kind of power.