If they decide to go with launch, we'll detect it and put mushrooms all over their front lawn, but we'll have enough in reserve to level anybody else who would take advantage of the situation. If it was China that launched against us, it is likely that we would have enough anti-missle, missles to bring down more than half of their 1st strike (they probably don't have a second strike scenerio due to a lack of warheads and vehicles. This is why I don't think that they would launch in the first place). This means we would suffer some "hits" on the west coast. Russia would stay out of this, because they could not benefit from the outcome. Any incoming warheads from anywhere would get a full retaliatory response, and then a second hit, just to make sure."Anti-Missile Missiles"? What are you talking about?
I know that Russia still has her "Galosh" Anti-Ballistic Missile defenses in operation around Moscow, but "Safeguard" (the US Anti-Ballistic Missile interceptor launch site) was shut down by Carter and the Democrats in 1978. (Besides which, the system was only designed to protect the US ICBM arsenal in Grand Forks, North Dakota -- it never had the range-capacity to knock down a Chinese nuke over LA or Seattle, anyway). The United States currently has zero capacity to knock down a nuclear-tipped Intercontinental Ballistic Missile -- not even one.
We have Patriots that can knock down a short-range SCUD moving at 3,000 miles per hour, but nothing that can even touch a Chinese ICBM moving at 16,000 miles per hour. If China launched all 20 of her ICBMs... all 20 would hit.
We're working on it, and we've got some good stuff going, but nothing dependably proven and deployed -- yet.
For now, we are still dependent upon MAD deterrence.
I know for a fact that Russia 'sold' one of its Galosh perimeter defence ABM missiles to the US so that they could derive some accelerated development from it. However the ABM missile the Russians sent over was a 'monkey model' since some of the more complex equipment had been removed, eg the mechanism to guide the kill vehicle to the terminal stage of the Ballistic projectile. So all we were stuck with was a nice booster stage rocket that was a Galosh, but lacked the real teeth of the actual missiles deployed around Moscow.