The Islamic State is indeed an 'existential threat' to the United States. By denying this, the only ones who are deceived is the American public, and, more importantly, those who are responsible for fighting this enemy—because it diminishes our capacity to understand the enemy for what it is, and what it is in the future capable of.  It diminishes our capacity to fight the threat, because we persistently, and deliberately, underestimate it out of pure vanity, ego, and overweening unjustified pride and arrogance.