It used to be said that terrorists were interested in activities that would have everybody watching, and not make everybody dead. In the intervening years, the threat has evolved, so that in the wake of 9/11, the Madrid and London bombings, and even the Aum Shinrikyo Sarin attack on the Tokyo subway system, we know that it may be merely a question of time before Al Qaeda, or any one of many ever-mutating groups seeks not only to acquire mass casualty weapons, but use them. By “WMD” (or mass casualty weapon), I mean specifically nuclear explosives, biological agents (including agents that attack plants or farm animals), chemical weapons, and the spreading of radioactive material.

Of these four plagues of modern civilization, radiological weapons are very likely to cause the fewest fatalities, and nuclear weapons almost certainly have the greatest potential for killing many people. Chemical and radiological weapons, as a rule, would likely reduce to mere nuisances by comparison with the probable consequences of using an advanced biological agent or, worse, a crude fission bomb such as that dropped on Hiroshima in 1945.