We’ve now passed the tenth anniversary of the 9/11 attacks, and widely expected acts of nuclear terrorism have yet to occur. One example: Graham Allison predicted in Nuclear Terrorism (2004) that, “In my considered judgment, on the current path, a nuclear terrorist attack on America in the decade ahead is more likely than not.”

We all know that the most likely form of nuclear terrorism is the use of a “dirty” bomb. There is no shortage of material for these instruments of mass disruption, and much of it is not well secured. As for an improvised nuclear device, we know about successful intercepts of small quantities of fissile material, but not about successful transactions. If there is opportunity – and presumed motive – why hasn’t a dirty bomb attack – or far worse – already happened?

John Mueller’s answer, in Atomic Obsession: Nuclear Alarmism from Hiroshima to Al-Qaeda (2010) is that nuclear dangers are far less than we presume:

Fears and anxieties about them, while understandable, have been excessive, and they have severely, detrimentally, and even absurdly distorted spending priorities while inspiring policies that have often been overwrought, ill conceived, counterproductive, and sometimes massively destructive. And they continue to do so.

Allison, and others who warn of impending disaster, may yet be proven correct. They maintain a well-defended position, whether they are right or wrong: by warning of tragic consequences, they have helped encourage remedial actions. Those issuing stark warnings can always argue that, as a result of their calls for preventative actions, worst cases haven’t occurred. It’s only fair that this line of argument – we’re right whether bad things happen or not – can be employed on behalf of arms-control measures as well as for hawkish remedies.

I periodically return to the subject of dogs that haven’t barked because answering the question, “Why not?” might help with peace of mind, and with peace and quiet, as well.

I’m not sold on the argument that extremists are highly motivated to use dirty bombs or to detonate mushroom clouds, even though there is evidence that Osama bin Laden was interested in acquiring a nuclear weapon. I question this post-9/11 conventional wisdom because I suspect that even extremists have to be cognizant of norms. If they blow by them, producing catastrophic damage, they could also alienate the very audiences they seek to champion. On the other hand, none of us can place too much faith in norms when dealing with those who carry out mass-casualty attacks.

A second possible reason for the absence of dirty bomb attacks – so far – is that they are unlikely to result in many casualties. Extremists have proven time and again that they can produce heavy loss of life with automatic weapons and conventional explosives. Why go to the trouble to acquire the material for a dirty bomb when more deadly instruments are so easily available? Nor do extremists need radiological material to get inside the heads of an entire population. Hell, one man and his surrogate son achieved this result in October 2002 by carrying out sniper attacks in the Washington, DC metropolitan area.

Another possible reason for the absence of dirty-bomb attacks might be their blow-back effects in the locations where they can most easily be carried out. If co-religionists are harmed more than foreign adversaries, extremists can lose followers. This argument would apply far more to bio-weapons and mushroom clouds than to dirty bombs. I’m not sold on this reasoning, either — even though Osama bin Laden seems to have embraced it in his lonely domicile in Abbottabad. The violence in Pakistan, Afghanistan, and Iraq has overwhelmingly been directed by Muslims against Muslims. These attacks may well breed resentment and lose potential adherents, but the slaughter continues.

Another possible reason for the absence of our worst, post 9-11 nuclear nightmares to date is that extremist groups may not have all the requisite skill sets, as was evident with Aum Shinrikyo’s failed attempts to employ bio weapons and nerve agents in Japan. The most worrisome case of BW terrorism so far has been the responsibility of a single, highly skilled, twisted individual with security clearances who worked at a US bio-defense laboratory. The problem with the skill-set argument: it doesn’t apply very well to dirty bombs.

Another possible reason for the absence of WMD terrorism or dirty bomb attacks has to do with organizational secrecy. To limit damages from exposure, extremist cells need to be kept separate. If cells and skill sets are separate, it’s harder to pull everything together without compromising operational security.

All of these possible answers have weaknesses, but taken together, they might help explain the absence of worst cases. Other answers seem clear-cut. Improved intelligence collection and sharing have surely helped prevent WMD terrorism, as have US initiatives to lock down dangerous weapons and materials.

Since all but my last two arguments are heavily conjectural, it makes good sense to keep focusing on – and funding – the last two.

Are we winning or losing the battle against proliferation? There are indicators that point in both directions. How you answer this question probably reflects your optimistic or pessimistic nature.

(See the first part of this post.)