You are pretty naive if you think that US cant be "taken over" by a hostile force.
It is not very likely that someone we know about today will ever try, because the costs heavy outweights the reward.
We never know what could happen in the future. Aliens invasion, goverments collapsing or a extrem powerful pandemic could of course change the setting very drastic.
I think that US is most likely the most well defended country on earth, but no one is safe from everything.