There is no value in fighting a traditional war for our own military. Not when we can devastate their assets from afar.
And if a situation ever occurred in which a war started to unfold this way, it would quickly be settled with nuclear weapons.
Also, if America ever started to lose a war on its own territory, there is no doubt what so ever that our leaders would unleash the entirety of our nuclear arsenal.
This is why America will never be conquered unless it is from within, by America itself.
It is sad but it is also true that most of us Americans view ourselves as the only people who matter. That being the case, we (as a Nation) would have no qualm what so ever at destroying all of mankind in the event that we were going down.
I, personally, do not agree with this mentality. But it is what it is.