For a country to invade the Mainland US would require a level of logistics that currently only the US Military could feasibly achieve.
An amphibious assault force on the West Coast would be extremely hard pressed to find a beachhead in which to land troops and material. The US Navy alone has multiple bases in Hawaii, Washington (State), and California which would screen any direct attacks against the coast (this is just US Navy; the US Army, Marines, and Air Force also have large presences in air bases, barracks, etc.)
Because of this, an invading force would likely either go North to establish a beachhead in Canada (our ally) and try to transport troops and equipment through rough/mountainous/forested terrain. Or, they would try to establish a beachhead in Mexico (also an ally) in which there aren’t very many deep water ports to handle the amount of long range troop transports necessary for an invasion.
Additionally there’s the issue of northern Mexico being a large desert where Air Superiority would likely be the deciding factor as any roads or railways would likely be taken out of commission which would slow an invasion to a crawl.
If an invader did either of those things, I do not for one second believe that the US Government would hesitate to strike those beachheads/ports/assets even with them being in foreign countries.
Air dropping paratroopers is unfeasible from across the span of an ocean and would require an invading force to be much closer to launch enough planes to have an effective fighting force, so a Red Dawn scenario is unlikely.
However, if it were going to happen that would put the adversarial country entering through Alaska or which ever islands in the Aleutian Islands could accommodate large enough air fields to launch an airborne invasion (which on its own would be useless without an amphibious assault establishing a beachhead at the intended target).
Assuming that an invasion force did land on the West Coast, it would have to contest with Active Duty assets, Reserve Assets, and the very likely possibility of civilian militias (Yes, even on the West Coast where civilian gun ownership is believed to be much lower).
On top of that, there’s the issue of terrain on the West Coast. A couple hundred miles East of the coast it gets extremely mountainous and rough before you get to the Plain States in which it is mostly flat and sparsely populated. Moving an invading force West to East would require a level of equipment, personnel, and readiness that no other country with exception of the US could muster (even if the PRC and Russia attacked jointly).
You want to invade from the East Coast? Same issues as the West Coast (although the terrain is slightly more forgiving). Scores of military bases with all of their equipment and personnel. Civilian gun ownership is also higher along the East Coast which would increase the possibility of civilian militias coming into contact with an invading force. If it were me planning to invade the East Coast/Gulf Sates, I would probably pick a launch point from Venezuela, but again, any Venezuelan assets used by an adversary amassing an invasion force would be taken out of commission before the invasion began without hesitation.