[Alpharius]

A sneak attack would be difficult and our navy and Air Force would give them hell before they hit the shore, where they'd hit the coast guard, marines, army and the national guard of each state they hit. Not to mention the rest of the 48 would probably be spending troops in, as well. While I don't doubt that a lot of vets and various police outfits would put up fierce resistance, your average joe blow with his glock wouldn't probably amount to much.

Even at the height of the Cold War neither Russia nor China seriously contemplated invading the US. The logistics of getting troops on the mainland, holding territory and supplying them would be a perpetual nightmare. Nuclear or biological weapons would need to be utilized. With nukes, they'd be destroying the infrastructure of the major cities and military installations - not to mention the grain belt where lots of our silos are, so no point in occupying after that. With biological weapons, you'd need something virulent and nasty, otherwise it's going be pretty localized and easy (relatively) to contain, if it is highly contagious, unleashing it in the US could backfire in a major way, being the transportation hub that it is.