A recurrent nightmare is gripping the American national security establishment. Leading politicians and counterterrorism officials in Washington are losing sleep over the so-called "lone wolf," allegedly the biggest terrorism threat of all.

The lone wolf is what "keeps me up at night," Michael Leiter, the director of the National Counterterrorism Center, recently told the Senate Homeland Security Committee. "It's quite clear," testified Lee Hamilton, the former chairman of the 9/11 Commission, "that the lone wolf terrorist has become a major threat and concern to the country." Central Intelligence Agency Director Leon Panetta acknowledged the same fear last winter to the Senate Intelligence Committee: "It's the lone wolf strategy that I think we have to pay attention to as the main threat." Senate Majority leader Harry Reid also told the Senate Homeland Security Committee that his "greatest concern" in life is the lone wolf.

Why are Americans charged with national security so afraid of the lone wolf? Since Sept. 11, individuals outside the chain of command of any established terrorist group have committed all four operationally successful attacks on American soil. The most notorious was by Major Nidal Malik Hasan, who never met a member of al-Qaida but frequented its websites before shooting and killing several of his Fort Hood colleagues in Texas in November 2009.

Lone wolves are hard for law enforcement to detect precisely because they do not openly associate with a terrorist group or its members. The Unabomber, Ted Kaczynski, was able to hide out as a hermit for 17 years in the mountains of Montana. And the 1996 Olympics bomber, Eric Robert Rudolph, eluded authorities for five years as a fugitive in the Appalachians. In its magazines and videos, al-Qaida is now urging the Muslim diaspora to become lone wolves, particularly unsuspected "clean skins" in the United States without traces to extremists. The Internet facilitates this clandestine strategy, as prospective lone wolves can now hop online, participate in a virtual jihadist training camp, and then carry out an attack under the nose of law enforcement. This lone wolf strategy is nearly unstoppable in democracies loath to turn their societies into police states.

But should the American public panic over this shadowy enemy? Is the lone wolf really so scary after all? Not if its record of lethality is any indication.

The four lone wolf attacks since Sept. 11 managed to kill just one civilian, a brave onlooker who bull-rushed Major Hasan with a chair before backup could arrive. Three of the four attacks — including that one — were actually cases of fratricide directed against fellow American soldiers. And the perpetrators used weapons no more powerful than a gun.

Historically, lone wolf misfires have greatly outnumbered massacres. Since the advent of international terrorism in 1970, none of the 40 most lethal terrorist attacks has been committed by a person unaffiliated with some terrorist group, according to publicly available data from the National Consortium for the Study of Terrorism and Responses to Terrorism, which is funded by the Department of Homeland Security and stored at the University of Maryland. In fact, lone wolves have carried out just two of the 1,900 most deadly terrorist incidents over the last four decades.

Instead of looking like Sept. 11, which required 19 men and other resources from the al-Qaida leadership, most lone wolf attacks kill just a single, unlucky bystander or nobody at all. American politicians and counterterrorism officials are particularly afraid of lone wolves operating within the United States. To date, however, such homegrown terrorists have taken just six Americans a year. Any deaths are too many, of course, but their low numbers from lone wolves hardly amount to a national nightmare.

Right before the outbreak of World War I, Sir Ralph Norman Angell published an unfortunate book, "The Grand Illusion," in which he notoriously predicted everlasting peace among the great powers of his day. Forecasting terrorism is even trickier because its practitioners aim to terrorize countries by striking seemingly at random. And yet, lone wolf attacks are nothing to lose sleep over if the past is prologue. If anything, the American public can rest assured that al-Qaida's growing reliance on lone wolves portends its organizational demise — from the post-Sept. 11 international backlash against its grizzly methods, Predator strikes on its core leadership, and endless string of political failures.

Max Abrahms is a Fellow at the Dickey Center for International Understanding at Dartmouth College and a faculty member at Dartmouth and the Johns Hopkins University, where he teaches courses on counterterrorism. His e-mail is maxabrahms@hotmail.com.