Understanding Terror Networks (2004) by Sageman, a senior fellow at the Foreign Policy Research Institute, is justifiably regarded as a must-read within the field. In contrast, Leaderless Jihad ultimately disappoints: Many of its conclusions tread old ground while its new conclusions are controvertible and frequently depart from the "scientific approach" that he advocates.
Sageman's idea of bringing the scientific method to the study of terrorism is a good place to begin understanding his work. The first chapter of Leaderless Jihad defends the scientific approach at length. He criticizes other authors for too much reliance on case studies since "basing conclusions on a single event or individual often leads them astray." In contrast, Sageman writes, a "scientific approach should encompass all the available data."
While Sageman's Understanding Terror Networks studied 172 terrorists, that sample grew to over 500 by 2008. Sageman shows that the majority of terrorists are not poor but middle class; that most terrorists are self-instructed in religion rather than trained in madrasas (Islamic schools); that the majority of his sample "held professional or semi-professional" employment positions; and that the terrorists he studied were well-educated. Rejecting mental illness and inherent criminality as explanations for terrorism, Sageman writes that terrorists are, generally, "neither bad nor mad." While his research on these points is solid, his conclusions are no different than they were in Understanding Terror Networks.
What is original in the 2008 volume is Sageman's analysis of what he dubs "leaderless jihad." He believes that the terrorist threat has "evolved from a structured group of al Qaeda masterminds … issuing commands" into "a multitude of informal local groups." In this new analysis, Sageman offers some decidedly non-empirical conclusions. For example, little empirical evidence supports his argument that "leaderless jihad" is a viable combat model. He concedes that moving to leaderless terrorism used to be "an admission of failure of traditional terrorism" but thinks that the Internet changes this. In fact, many plots that fit the leaderless jihad model—such as those in Fort Dix or Miami—have been characterized by operational incompetence compared to those where Al-Qaeda's central leadership has had a hand organizing the plot or providing training.
Similarly, Sageman's policy prescriptions contain questionable claims. He discounts the need for a coherent response to Al-Qaeda's religious ideology on the basis that he has "traveled to several trials of terrorists in Western Europe, spoken to people who knew them as children and as young men, and read the open-source literature about them." These observations prompt him to conclude that "the terrorists in Western Europe and North America were not intellectuals or ideologues, much less religious scholars." Sageman thus warns against "over-intellectualizing" the fight against jihadist ideology. This conclusion falls short of Sageman's plea for an approach that examines "all the available data": He presents no concrete data and offers no reason to believe that the trials he studied are representative.
Sageman calls for rapid U.S. withdrawal from Iraq, stating that the "presence of even one American soldier in uniform in Iraq will trump any goodwill policy the United States attempts to carry out in the Middle East." Though the Iraq war has been a boon to jihadist recruiting, his claim is not supportable. Aside from the hyperbolic assertion that a single American soldier in Iraq would undermine any U.S. goodwill policies, his prescription ignores the fact that Iraq has been costly for jihadists since early 2007. Doesn't Washington gain something by leaving behind a relatively stable country, as opposed to the propaganda victory that jihadists would gain if the United States withdrew in defeat?
This volume lacks the rigor of Understanding Terror Networks, too often departing from scientific methodology or reaching questionable conclusions.
 Marc Sageman, Understanding Terror Networks (Philadelphia: University of Pennsylvania Press, 2004).