Hippophibia, the problems (for Russo-China) with that scenario, are:
1. The US has what is likely a decent defense system vs. ballistic missiles (I say likely as it has never been fully tested of course). By no means full-proof, but effective (based on limited testing results). In addition, there are WAY more targets needed to take out in the US to cripple it, than there are in Russia or China (meaning our missles will be more effective). Do you think US missiles will sit in the silos? Not to mention all those subs and bombers.
2. Russia and China simply don't have the vehicles to get here with enough infantry (and stealth) to be successful with a land invasion. Couple this with an armed citizenry, and an invasion of the US is simply a logistical nightmare for any other military on the planet. Even if they could manage it initially, it would be via a VERY susceptible supply train that could easily be cut off.