The thought that terrorists could successfully detonate a nuclear bomb devastating one of the world's great cities seems incredible, even inconceivable. Psychologically, it is almost impossible to internalize an act of terrorism that kills not just 3,000 human beings as on 9/11 but 300,000. Analytically, however, the stepping stones from 9/11 to a nuclear 9/11 are readily traceable—now with a readable guide in Michael Levi's On Nuclear Terrorism.

Before the September 11, 2001, assault on the World Trade Center and the Pentagon, who could have imagined that terrorists could mount an attack on the American homeland that would kill more citizens than Japan did in the Pearl Harbor attack that brought the United States into World War II? As Condoleezza Rice testified to the 9/11 Commission, “No one could have imagined them taking a plane, slamming it into the World Trade Center, using planes as missiles” (New York Times transcript of the testimony of the national security adviser, Condoleezza Rice, before the National Commission on Terrorist Attacks Upon the United States, April 8, 2004; available at: http://www.nytimes.com/2004/04/09/us/threats-responses-excerpts-rice-s-testimony-before-commission-investigating-sept.html?scp=1&sq=&pagewanted=all).

As is now painfully evident, assertions about what is “imaginable” or “conceivable” are propositions not about what is possible in the real world but about fallible human minds. After examining all of the evidence about what the U.S. government did, and failed to do, prior to 9/11, the major conclusion of the commission is that the principal failure of the U.S. national security establishment was a “failure of imagination.”

Shortly after 9/11, CIA Director George Tenet flew to Pakistan to enlist President Pervez Musharraf's assistance in preventing the next attack, one that he warned could be a “nuclear 9/11.” Musharraf reacted in disbelief: “men in caves can't do this” (George Tenet, At the Center of the Storm: My Years at the CIA, 2007, pp. 266–67). Strange as it seemed, Tenet explained, if the Al Qaeda terrorists who executed 9/11 had acquired highly enriched uranium made by a state like Pakistan, they could indeed have constructed an improvised nuclear device capable of killing hundreds of thousands of people.

President Barack Obama has argued that “the single biggest threat to U.S. security in the short, medium, and long term is the possibility of a terrorist organization obtaining a nuclear weapon” (statement by the president at the April 12–13, 2010, Nuclear Security Summit; available at: http://www.politico.com/politico44/perm/0410/the_single_biggest_threat_to_u_s_security_eabaf67c-c4bb-4155-81f7-75afd3af3e49.html). As chair of the UN Security Council in September 2009, he challenged member states to consider the impact of a single nuclear bomb: “Just one nuclear weapon exploded in a city—be it New York or Moscow, Tokyo or Beijing, London or Paris,” he noted, “would destabilize our security, our economics, indeed our way of life” (President Obama's speech at the UN Security Council Summit on Nuclear Non-Proliferation and Nuclear Disarmament, September 24, 2009; available at: http://www.cfr.org/publication/20277/obamas_speech_at_the_united_nations_security_council_summit_on_nuclear_nonproliferation_and_nuclear_disarmament_september_2009.html).

Levi advances our understanding of this ultimate threat and illuminates the shape of the problem for policymakers and the broader public. In providing one of the most comprehensive accounts of how a terrorist group could acquire, deliver, and detonate a nuclear weapon, he demonstrates that this nightmare scenario is a real and present danger. Hie analysis identifies key decision nodes on a terrorist group's nuclear odyssey. For each, he analyzes specific hurdles and opportunities. In contrast to abstract “worst case scenarios,” he urges policymakers to focus on tactics and plots commensurate with each group's capabilities.

Levi notes that there are three basic challenges that a terrorist group must overcome in order to execute a successful nuclear terrorist attack: 1) acquiring fissile material or a nuclear weapon, 2) constructing or activating a bomb, and 3) delivering the bomb to its target. Each of these poses a number of technical, logistical, and financial obstacles. The flip side of each hurdle faced by terrorists is an opportunity for preventers to make these challenges even more daunting and success even more uncertain. So, for example, the author recommends that policymakers conduct black market sting operations in order to deter terrorists from seeking nuclear materials, thus increasing their perceived risk of future failure. A well-known version of this approach in the area of illicit drugs “muddies the waters” by introducing contaminated or fake material into the market to frustrate gullible buyers. On at least three occasions, Osama bin Laden has bought nuclear material that turned out to be unusable for a nuclear bomb.

History offers an even more imaginative example in the case of “red mercury,” a substance allegedly created by the Russians in the 1980s to fashion miniature nuclear bombs. Its price reportedly ranged from $100,000 to $300,000 per kilogram on the black market, attracting a number of eager buyers (see Rensselaer Lee, “Smuggling Update,” Bulletin of the Atomic Scientists [May/June 1997]: 52–56). It is now well known that this substance never existed.

Levi presents a coherent argument for a layered system of defense, emphasizing ways of enhancing, or even exaggerating, would-be terrorists' fears that their project could fail. In emphasizing the complexity of the terrorists' challenge, and the hurdles and risks of failure at each step, he avoids simplistic errors like the traps that snare John Mueller. After identifying 20 steps an atomic terrorist would have to accomplish for success, Mueller concluded in Atomic Obsession: Nuclear Alarmism from Hiroshima to Al-Qaeda (2009) that there is a “worse than one in a million” chance that a given nuclear terrorist attack would succeed. He reached this conclusion by treating each barrier as “effectively independent” from the next, assuming that the terrorist has a 50% chance of failure at each, and multiplying those 20 probabilities together. His approach, however, misinterprets probabilistic risk assessment. For example, some of the steps on the list would have to be completed before an attempt to acquire material could begin (therefore, the success rate for any of those steps during the path would, by definition, be 100%). But in attempting to turn on its head the conventional wisdom that defenses have to succeed all the time, unlike terrorists who have to succeed only once, he misses the mark. Only if terrorists' failure at one hurdle leads them to abandon the undertaking could this be correct. As Rolf Mowatt-Larssen demonstrates (in “Al Qaeda Weapons of Mass Destruction Threat: Hype or Reality?” paper presented at the Belfer Center for Science and International Affairs, Cambridge, MA, 2010), despite repeated failures, including multiple unsuccessful attempts to buy bomb-usable fissile material, Al Qaeda has persisted.

In discussing what most serious analysts agree is the single most effective layer of prevention—mainly, denying terrorists the nuclear means to achieve their deadliest ambitions—Levi dismisses this goal as a “myth.” But the evidence and analysis he presents will not persuade many readers—and should not. Former Senator Sam Nunn (D-GA) has stated the central truth: The point at which defenses are easiest for the preventers to erect and hardest for the terrorists to penetrate is at the source. Every mile from that point, the challenge becomes easier for the terrorists and harder for the preventers.

As Matthew Bunn has observed about On Nuclear Terrorism (“It's Worse Than You Think,” Talking Points Memo, December 6, 2007), the fact that “only one paragraph” discusses the next steps in securing nuclear stockpiles at the source makes it seem as though all defensive measures are equal. They are not. As a matter of physics: no fissile material, no mushroom cloud, no nuclear terrorism.

Recognizing this fact, President Obama assembled 46 world leaders in April 2010 for the first Nuclear Security Summit as part of an aggressive effort “to secure all nuclear weapons materials at vulnerable sites within four years” (remarks by President Obama in Prague, Czech Republic, April 5, 2009; available at: http://www.whitehouse.gov/the_press_office/Remarks-By-President-Barack-Obama-In-Prague-As-Delivered/). The big insight that he attempted to drive home is that these leaders are capable of preventing the only terrorist attack that could kill hundreds of thousands of individuals in a single blow. What the members of the international community have to do to prevent this ultimate catastrophe is lock up all nuclear weapons and materials as securely as gold in Fort Knox or treasures in the Kremlin Armory. Achieving this goal is not impossible, as Levi claims, but in fact consists of actions that we know how to take and can afford.

Levi's analysis also overstates the technical obstacles a terrorist group would confront in attempting to construct a bomb. His discussion on recruiting a cadre of skilled technicians, fashioning combustible material into a bomb-usable source, and building an initiator almost suggests that terrorists would need the resources of a state to succeed in their task. Unfortunately, the consensus judgment among experts in disagreement is correct. As John Foster, a leading American bomb maker and former director of the Lawrence Livermore National Laboratories, wrote three decades ago, “If the essential nuclear materials are at hand, it is possible to make an atomic bomb using information that is available in the open literature” (quoted in Allison, Graham, Nuclear Terrorism: The Ultimate Preventable Catastrophe, 2005, p. 10). Examining all the evidence about this critical question, the Commission on the Intelligence Capabilities of the United States Regarding Weapons of Mass Destruction, known as the Silberman-Robb Commission, reported in 2005 its conclusion that “fabrication of at least a ‘crude’ nuclear device was within Al Qaeda's capabilities, if it could obtain fissile material” (report available at: http://www.gpoaccess.gov/wmd/index.html).

In the oft-quoted observation of Harold Agnew, another American bomb maker and head of a weapons lab, those who believe that it would be easy for terrorists to make an elementary nuclear bomb are wrong. But those who believe that it would be impossible for terrorists who acquired fissile material to make an elementary nuclear bomb are dead wrong.

On the bottom-line question—Is nuclear terrorism a real threat?—Levi answers “yes.” By raising the consciousness of leaders and officials about the specific threat of nuclear terrorism, and highlighting opportunities for policymakers to make significant improvements in preventing terrorists from achieving their deadliest ambitions, On Nuclear Terrorism makes an important contribution to the growing body of policy-relevant scholarship on the subject.