The United States imperial presence in the Middle East is the only thing threatened. Yes, there may be attacks on U.S. territory used as a tactic to drive us out.
9/11 occurred because jihadists want the U.S. out of the Middle East, not because they hate us intrinsically. Though some certainly do, it is our bullying in the region that inflames their leaders and opens the average person's mind to the idea of fighting against the U.S.
ISIS is only a threat to the U.S. because the U.S. won't leave the region alone.
Empire does not work. Unfortunately, simply leaving the region won't 'fix' things now.
There is a threat, but no more a threat than any other country would face if it had invaded and occupied the United States for 50+ years.