We will never have to find out because we have the greatest defense and offense the world has ever seen. We have the largest confirmed nuclear arsenal on the planet and if we fall the whole human race will follow(I’d hope at least). It wouldn’t even come to that though, america is the only nation on earth capable of sustaining over seas conflict, our military evolved differently during world war 2 than the rest of the world’s. Closest to our military which still isn’t close at all would be the British empire’s. Even then they didn’t have the same capacity as our military did almost a hundred years ago and today’s absolutely dwarves theirs.
We have the most military infrastructure around the world, the most allies, the most staging points for invasions, any government that attempted to take America in a direct assault would find themselves dead in the pacific or Atlantic before even seeing our shores.
All that aside, yeah I own a rifle and would love to shoot some invaders. I think most Texans feel that way.