Well, the US is situated in a very unique way, we have two exposed coasts on opposite sides of the country, both are very long coasts with thousands of possible invasion sites, but no other nation in the world really has the ability to hit both at once, we on the other hand can use the Panama Canal and our extensive interstate system to transport almost our entire armed forces from coast to coast in a matter of weeks, maybe even days. Either way surprise is not very easily achieved, our Navy and Air Force would have to be destroyed, meaning that the UK, Russia, Japan, and a couple other European Nations would have to be involved because our Navy and Air Force are second to none, our Marines Corps. or Army might have their equals in the UK, Russia, China, Germany or France but even those above mentioned countries Navies combined would not be able to take out our Navy AND Air Force, we have 14 fleet carriers, 13 more than any other nation. Russia might have a large fleet of submarines but I think the Kirsk incident proved the state it is in.
So, if you have a two front invasion you need to have a coalition of nations devoting all their resources to destroying the US, and don't forget about militias. Either way the Air Force adn Navy will have to be destroyed, Hawaii would have to be invaded first if coming from Asia and Canada would make a much better back door than South America. If it is China invading set it in the year 2097, Russia 2068, and the UK better go with 1862, Brazil invading, forget about it, France? Nope, Germany? Even less likely.