internal conflict will end the US, not foreign attacks
the US is a very, very, VERY difficult nation to invade, and with the advancement of AA weaponry and the existence of mutually assured destruction, nuclear attacks are not an option unless you're willing to destroy the entire planet
back to the difficulty of invading it, though:
it shares only two land borders, one with a long standing ally and another with mexicans that tend to keep invading themselves rather than their surroundings (seriously,
look at how many wars they've been involved in were just internal strifefurthermore, guess where one of the largest ever US forts is? texas. it's fort hood.
it has the largest naval presence on the planet. the coast guard is bigger than the british royal navy.
its civilian population has forget tons of guns.
it is geologically hard to occupy due to its size.
russia and china are both dealing with some pretty significant problems right now, by the way, so there's that.
