I mean, to be fair, this is the actual situation with nuclear weapons.
There's nothing you can do. You just... die. Everyone. There's no magic bullshit the US can pull out of it's ass to prevent it. One singular nuclear warhead can be enough to eradicate the entire population in the US.
How big would this warhead be? "One singular nuclear warhead" wasn't enough to eradicate the entire populations of Hiroshima and Nagasaki, much less Japan.
If you think that wouldn’t happen, then you have to believe the US would accept a nuclear strike on one of its population centers and respond with conventional force and/or law enforcement. I am not confident!
Modern nuclear strategy does not aim to use nuclear weapons to immediately kill everybody in the US. We moved away from really big payloads towards smaller independent payloads with better targeting precision, along with greatly reducing the size of the stockpile.