The terrorist attacks of September 11, 2001, brought together two related but distinct phenomena. First, they presented the calling card of al Qaeda and more generally of militant Islam. These attacks were rightly perceived as an act of war by a group seeking to catalyze a political-religious movement. Much of America's effort since then has been to destroy that group, its sanctuaries, and its affiliates; some of the effort has been to counter the psychological, social, and political appeal of militant (predominantly Wahabi) Islam. Second, these attacks introduced the public to a more general phenomenon: our vulnerability to acts of terror on a greater scale than anything America had experienced. It is remarkable that in the turbulent 20th century, which witnessed some 200 million deaths from politically driven violence and war, no single attack on American soil equaled the estimated 3,000 deaths on 9/11. The implications for America are the graver because the capability to inflict carnage at this level-and at much higher levels-is not confined to a group or movement. It lies at hand as an instrument that can be used by any belligerent group (or state, or individual). It will survive the destruction of al Qaeda and the abandonment of jihad. These two strands-jihadi terrorism and our general vulnerability to terror on a large scale-intertwine but are independent. The tendency to confuse them is accentuated when policymakers rhetorically jump from one to the other; the effect resembles one produced by the thaumatrope, a popular 19th-century toy now encountered only as a curiosity. A horse is depicted on one side of a disk or card and a man on the other, or a cage on one side and a bird on the other. When the object is spun quickly, the rider appears on the horse, or the bird in the cage. Our inability to separate images shown us in rapid succession merges the two in our minds. We speak of a "war on terror" (not just on al Qaeda) and have devoted significant resources to controlling and preparing for the consequences of "weapons of mass destruction," but these efforts overwhelmingly focus on the present challenge of jihadi fundamentalism. Our inherent vulnerability to large-scale terrorism is more troubling but less addressed. Our vulnerability to the use of chemical, biological, radiological, nuclear, and other technologies to create terror is a risk. These technologies provide instruments that can be seized upon by any group for use as weapons of terror. So long as grievances exist and those who hold grievances are willing to resort to violence, the use of such weapons will be an enduring risk. The sweep of history suggests that these risks cannot be eradicated. Apart from the dum-dum bullet, we cannot point to examples of effective weapons that have not been used. Once used successfully, weapons tend to proliferate. That proliferation is abetted when the skills that can produce a weapon are closely related to civilian skills and equipment that are themselves proliferating. To cope with our inherent vulnerability to weapons of terror, we must find strategies of risk management.