yea what femme Fatale said... it's a logistic nightmare if an enemy wants to invade America because they'll have to deal with both Canadian and American military. If you look at the map to see how they can mobilize tanks and armies into America from Alaska thru a road... I say GOOD LUCK! the nature and weather will take care of it for us.
that's a beauty about America... it's a logistic nightmare for any enemy to invade this country. that's why the American Revolutionary War dragged on for 8 years. and it took America approximately 6 months to mobilize everything to 1991 Persian Gulf War.
It would be much easier and cheaper to invade America thru shores with aircraft carriers or whatever but then... good luck surviving our counterattacks while crossing the ocean! It would make Normandy landing looks like a small skirmish 
