Could America be invaded... eventually ? Yes.
Could the United States be invaded today or in 20 years ? No.
The problem isn't the geography of the land, the amount of resistance put up by the soldiers, it's simple numbers. How could any armed forces get close enough to even strike the United States with anything that isn't an intercontinental ballistic missile ? (not counting Mexico and Canada)
Well, it's impossible because the United States have something that is called the US Navy, and which is the largest fleet in the World, by far. And the best fleet in the world in case you were wondering.
And except if the US Navy was lead by a bunch of idiots, there is no way they would let an ennemy fleet get close enough to the US coast to invade or even strike it.
Even an invasion in Alaska would be easely repelled with the amount of forces stationned there and the possibility of sending an carrier group within a few weeks.
The scenario in World in Conflict is nice to see but could not happen. Even if soviet forces were hidden in civilian boats, the US Air Force would quickly take care of both the troups and the boats, while the US Navy would make sure that no additionnal forces could approach the coast. Because in that case, the US forces would still have a huge air and naval superiority that would enable them to take care easely of any ground forces.
As long as the US Air Force still has bases all over the country, and as long as the US Navy still rules the see, an invasion of the United States is impossible.
By the way, in case you were concerned by my over-confidence in the United States military, I'm french.
In France, we have another strategy in the case of a real invasion of the territory (besides taking care of it with our forces stationned in the country) : The use of a nuclear weapon on the attacker.
