If CCP ICBMs were detected heading our way - THAT’s a declaration of war!
China does even need to launch missiles. China has about 200 missile silos under construction. Those missiles will have multiple, independently targetable warheads. Let’s use 1000 warheads in total as a very conservative estimate. (The real number is more like ten warheads each.) The US has about 150 Minuteman silos, with each missile limited to a single warhead due to an old treaty with the Russians. We have some submarines with additional missiles, but they have an unknown number of surface ships with missiles.
Faced with those odds, the current political and military leadership will surrender whenever the Chinese demand it, to avoid total destruction.
But like Jonty 30, I think they will use an EMP. They will want the US land mass primarily for farming, and EMPs will not damage the resources. They might even use an EMP AFTER we surrender. (That’s great book by the way.)