system
A chat on subreddit /r/NoStupidQuestions
human
It's impossible to invade because it's located on a separate continent, has essentially two neighbors, neither of which can be realistically used for planned attack (as everyone will know) and no one has navy big enough to do naval invasion (you need to destroy US Navy and the transport gazillion of troops and equipment). 

Then even if you somehow get the coasts, you'll get to natural barriers (Appalachia, Rockies), probably winter will start (usually cooler than in Europe) and you'll have hard time to control the area due guns everywhere. Private guns won't matter during military phase but they really matter when deciding how hard and expensive is maintaining control.
gpt
The sheer size of the country and wide distribution of the population and government adds to the occupation problem.