The only nations that possess both the technology and manpower to invade the USA (India, China) do not have anything like the logistical requirements. Until Star Trek style transporters are invented and become commonplace nor will they have either.
I think I can safely say that the USA is pretty much immune to a military invasion at the present time and for the foreseeable future.
"An Emperor is subject to no-one but God and justice."
Frederick 1, Barbarossa