If anyone else has ever tried to invade another continent on a fair technological ground on civilization 4, you will realize that it's impossible.
Even with a VAST troop advantage, the fact is that you are surrounded by the enemy with little room for escape, no room for error and money flowing from your pockets so fast that there is no way you could prevent economic downfall.
You are assaulting a continent in which you have no friends anywhere nearby. (Canada would fall into a puddle of our own urine if we were not standing beside the united states, and mexico is a bit of a non-issue), a continent posessing more guns, fanaticism and ignorant prejiduce than any other continent in the world, and a people with a reckless 'just nukem' attitude. (Sorry, this isn't meant to be anti-american, I'm proving a point).
The surrounding area is water. Just water. Nothing but water. Ships are a relatively slow, and inconvenient method of travel, easily engaged and difficult to hold, and air travel into the united states is effectively impossible.
The fact is, the US could never be invaded. The US is not just a country, it's effectively a continent, surrounded by water, well armed, easily defended, even by a shattered military and zealous and patriotic citizenry.