Haha, I think you watch too much Hollywood. No one has invaded the US because there was no value in doing so. Germany taking over most of Europe made sense. Taking over the US? Not so much. At least not until they had a much larger land holding.
The US' geographical position has had more to do with their safety than any secret they've ever had. But don't let me disrupt your "G.I. Joe" comic book fantasies.
Why do you think those answers are what they are?
Could it be that the latest public data we have is that we were building planes FIFTY YEARS AGO that could effortlessly outrun SAMs? Can you possibly imagine the shit that is flying out of Groom Lake these days?