First, intelligence sources would pick up that an invasion force was building up through spys, satellite imagery and listening posts around the globe. Even if somehow by a miracle nothing was ever known of this event the geographical isolation that has served us so well over the centuries would again help our cause. It would take a very long time for any appreciable force to arrive on our shores in which time they would be detected and trigger a reaction.
Secondly, our allies such as Canada, Great Britian and Austraila, to name but a few, would come to our aid. No foreign country at this point has a military force strong enough to fend off the combined powers of our allies. Now, in this scenario if we were to believe that one of the aforementioned countries was to initiate the invasion which in itself is preposterous, they would still face the other factors I mentioned.
Third, never underestimate the power of the people. Think back to Afghanistan, Vietnam's Viet Cong, partisan fighters in WWII and currently Iraq. Controlling all of the continental US would be all but impossible and provide countless hiding places for rebel forces to operate from and fight. History has shown a determined force fighting for their homeland can defeat a foreign invader.
Ultimately the US will crumble like all great powers have over history but at this time we are the most powerful nation on Earth and it's unthinkable that an invader could occupy our lands.