they would have to work with us to set up a similar set of interceptor sites across the northwest territories, but they are largely defended from attack in the mainland us by the depth and size of the canadian frontier sitting in front of it.
i clearly remember when george w. bush tried to push for missile defence - before the israelis proved it was possible - he showed a map demonstrating how the system would knock down incoming nuclear missiles that had the debris making a direct hit on an area in canada called EDMONTON, which was unmarked on the map, and which the americans doing the presentation were entirely oblivious of. they had no idea. needless to say, paul martin wasn't very excited about this.
it would make more sense to do this through norad and extend the shield through the canadian rockies into alaska, and then across the north to greenland. maybe something falls on baffin island. i wish no harm on the inuit, but we can't save everybody and we should be realistic about that.
but a shield across the continental american states? i don't think that's such a hard engineering problem at all. this is the country that built the hoover dam and beat the nazis to the bomb. c'mon guys. rediscover yourselves.