This article aims to explain why the idea of the West is, for historical and philosophical reasons, an obstacle to dealing with the dangers posed by radical Islamists. Every proposed theory of the West has to account for the great internal cultural diversity both of European cultures and of those influenced by them around the world; and every serious historical account both of Europe and of Islam has to recognize the long-standing, substantial and ongoing interdependence of their intellectual and religious traditions. As a result, what is needed to face extremists, whether inside or outside Europe (and whether Christian, Muslim or neither), is not an opposition between Islam and the West, but an alliance of those of all faiths and none who can live with and tolerate cultural difference against those, wherever they live and whatever their religion, who cannot.