The US has with no doubt the best ground on the planet to defend themselves with. Isolated by two large oceans and bordered by "mostly" friendly nations

Force extended requires significant amplification.

So for China to impact us in a meaningful way they would have to somehow sneak a force large enough to damage across a pretty wide open space, then maintain that logistically.

Ditto for the RU

That said once in the US you have 300 million small arms manned by 21+ million veterans in the US

Not sure China can actually get that many boots on the US soil before the Navy / Air get a poke or two at them