And, a Canadian invasion? Not very likely, two states worth of hard core patriots would pretty much lock up any northmen and make any attempts to invade south a very poor idea! Mexicans, poorly armed and equipped with a mix mash of equipment, ammo and weapons, and little if any logistics to mount any thing more then gorilla actions! Mex Mil, A bad joke at best! A head ache at worse, and short lived! Chine or Russia, very unlikely, neither has sufficient transports to move large numbers of fighting men and equipment across the seas in full view of the worlds most powerful Navy ( still fully capable of taking on any TWO of the worlds finest at the same time) and it's buddies in the Coastguard and even Marines, and within range of the Air Force, noting but flotsam would reach our shores! A U.N. invasion would be the exact same, nothing but tide and current swept floaters reaching out shores!