25 years ago I would have said that any country invading America would be heading into a meat grinder. Admiral Yamamoto, the architect of the Pearl Harbor attack stated as much to the Japanese big wigs not to contemplate a land invasion of America. "There will be a rifleman behind every blade of grass."
Now, with the wussification of men, over liberalization of women, I am not so sure enough would fight to do anything.
Oh yeah. The wetbacks would turn tail and head south as fast as possible, grabbing as much of your shit as they could carry on the way out.