The credibility of the nuclear disarmament movement has been renewed in recent years. Credit the famous “four horsemen” opinion pieces in the Wall Street Journal, the Prague speech in President Barack Obama, or the “rebirth” of arms control negotiations and the New Start Treaty. Whatever the cause, the prospect of “a world without nuclear weapons” is no longer dismissed as mere fantasy. The path to nuclear zero will be difficult; it will remain beyond reach if we don’t fully appreciate and understand the real obstacles to its realization.