The only way I can see that the U.S. could be invaded is if an enemy managed to strike a deal with Mexico and/or Canada, maybe arm them with more soldiers/weapons, and they launch a suprise attack.
Please tell me if that sounded illogical/stupid.
Actually, if they weren't very close allies, they would probably do that in exchange for the spoils of the invasion.
It almost happened in World War 1 with the Zimmerman Note/Telegram. Fortunately, Mexico declined the German invitation because Mexico was dealing with a civil war, and they couldn't have staged an attack on us.