I find it unlikely that many modern Americans would have the balls to stand up against an invading army, considering how spinless and easily persuaded we’ve shown ourselves to be in reference to our domestic political bullies. But you never know. The French sure gave the Nazis a hell of a time while they were occupied.
Fortunately, (as of now) too many foreign markets rely on our economy. So it’d be bad business to invade us, or at least enough countries would feel the same and cone to our defense.
Money speaks an international language.
P.S. For anyone interested in this subject, there was a (kinda cheesy) movie called Red Dawn about the USSR invading a town in Colorado or somewhere. Gotta love political dogma from the early 80’s.