A few days after the September 11th attacks—which killed seven times as many people as any previous act of terrorism—President George W. Bush declared that the United States was engaged in a global war on terror. September 11th seemed to confirm that we were in a clash of civilizations between modernity and radical Islam. We had a worldwide enemy with a cause that was general, not specific (“They hate our freedoms”), and we now had to take on the vast, long-running mission—equal in scope to the Cold War—of defeating all ambitious terrorist groups everywhere, along with the states that harbored them. The war on terror wasn’t a hollow rhetorical trope. It led to the American conquest and occupation first of Afghanistan, which had sheltered the leaders of Al Qaeda, and then of Iraq, which had no direct connection to September 11th.

Game theorists posit that even suicide bombers are rational actors. Illustration by Francesco Bongiorni

Today, few consider the global war on terror to have been a success, either as a conceptual framing device or as an operation. President Obama has pointedly avoided stringing those fateful words together in public. His foreign-policy speech in Cairo, last June, makes an apt bookend with Bush’s war-on-terror speech in Washington, on September 20, 2001. Obama not only didn’t talk about a war; he carefully avoided using the word “terrorism,” preferring “violent extremism.”

But if “global war” isn’t the right approach to terror what is? Experts on terrorism have produced shelves’ worth of new works on this question. For outsiders, reading this material can be a jarring experience. In the world of terrorism studies, the rhetoric of righteousness gives way to equilibrium equations. Nobody is good and nobody is evil. Terrorists, even suicide bombers, are not psychotics or fanatics; they’re rational actors—that is, what they do is explicable in terms of their beliefs and desires—who respond to the set of incentives that they find before them. The tools of analysis are realism, rational choice, game theory, decision theory: clinical and bloodless modes of thinking.

That approach, along with these scholars’ long immersion in the subject, can produce some surprising observations. In “A Question of Command: Counterinsurgency from the Civil War to Iraq” (Yale; $30), Mark Moyar, who holds the Kim T. Adamson Chair of Insurgency and Terrorism at the Marine Corps University, tells us that, in Afghanistan, the Taliban’s pay scale (financed by the protection payments demanded from opium farmers) is calibrated to be a generous multiple of the pay received by military and police personnel (financed by U.S. aid); no wonder official Afghan forces are no match for the insurgents. Audrey Kurth Cronin, a professor of strategy at the National War College, reminds us, in “How Terrorism Ends: Understanding the Decline and Demise of Terrorist Campaigns” (Princeton; $29.95), that one can find out about Al Qaeda’s policy for coördinating attacks by reading a book called “The Management of Barbarism,” by Abu Bakr Naji, which has been available via Al Qaeda’s online library. (Naji advises that, if jihadis are arrested in one country after an attack, a cell elsewhere should launch an attack as a display of resilience.) In “Radical, Religious, and Violent: The New Economics of Terrorism” (M.I.T.; $24.95), Eli Berman traces the origins of the Taliban to a phenomenon that long preceded the birth of modern radical Islam: they are a direct descendant of the Deobandi movement, which began in nineteenth-century India in opposition to British colonial rule and, among other things, established a system of religious schools.

What is terrorism, anyway? The expert consensus converges on a few key traits. Terrorists have political or ideological objectives (the purpose can’t be mere profiteering). They are “non-state actors,” not part of conventional governments. Their intention is to intimidate an audience larger than their immediate victims, in the hope of generating widespread panic and, often, a response from the enemy so brutal that it ends up backfiring by creating sympathy for the terrorists’ cause. Their targets are often ordinary civilians, and, even when terrorists are trying to kill soldiers, their attacks often don’t take place on the field of battle. The modern age of suicide terrorism can be said to have begun with Hezbollah’s attack, in October of 1983, on U.S. marines who were sleeping in their barracks in Beirut.

Once you take terrorists to be rational actors, you need a theory about their rationale. Robert Pape, a political scientist at the University of Chicago, built a database of three hundred and fifteen suicide attacks between 1980 and 2003, and drew a resoundingly clear conclusion: “What nearly all suicide terrorist attacks have in common is a specific secular and strategic goal: to compel modern democracies to withdraw military forces from territory that the terrorists consider to be their homeland.” As he wrote in “Dying to Win: The Strategic Logic of Suicide Terrorism” (2005), what terrorists want is “to change policy,” often the policy of a faraway major power. Pape asserts that “offensive military action rarely works” against terrorism, so, in his view, the solution to the problem of terrorism couldn’t be simpler: withdraw. Pape’s “nationalist theory of suicide terrorism” applies not just to Hamas and Hezbollah but also to Al Qaeda; its real goal, he says, is the removal of the U.S. military from the Arabian Peninsula and other Muslim countries. Pape says that “American military policy in the Persian Gulf was most likely the pivotal factor leading to September 11”; the only effective way to prevent future Al Qaeda attacks would be for the United States to take all its forces out of the Middle East.

By contrast, Mark Moyar dismisses the idea that “people’s social, political, and economic grievances” are the main cause of popular insurgencies. He regards anti-insurgent campaigns as “a contest between elites.” Of the many historical examples he offers, the best known is L. Paul Bremer’s de-Baathification of Iraq, in the spring of 2003, in which the entire authority structure of Iraq was disbanded at a stroke, creating a leadership cadre for a terrorist campaign against the American occupiers. One of Moyar’s chapters is about the uncontrollably violent American South during Reconstruction—a subject that a number of authors have turned to during the war on terror—and it demonstrates better than his chapter on Iraq the power of his theory to offend contemporary civilian sensibilities. Rather than disempowering the former Confederates and empowering the freed slaves, Moyar says, the victorious Union should have maintained order by leaving the more coöperative elements of the slaveholding, seceding class in control. Effective counterinsurgency, he says, entails selecting the élites you can work with and co-opting them.

In “Talking to Terrorists: Why America Must Engage with Its Enemies” (Basic; $26.95), Mark Perry describes a little-known attempt to apply Moyar’s model in Iraq. The book jacket identifies Perry as “a military, intelligence, and foreign affairs analyst and writer,” but his writing conveys a strong impression that he has not spent his career merely watching the action from a safe seat in the bleachers. Much of the book is devoted to a detailed description, complete with many on-the-record quotes, of a series of meetings in Amman, Jordan, in 2004, between a group of Marine officers based in Anbar province, in western Iraq, and an Iraqi businessman named Talal al-Gaood. Gaood, a Sunni and a former member of Saddam Hussein’s Baath Party, suggested he could broker a deal that would make the horrific, almost daily terrorist attacks in western Iraq go away.

Perry’s tone calls to mind a Tom Clancy novel. Tough, brave, tight-lipped officers do endless battle not just with the enemy in the field but also with cowardly, dissembling political bureaucrats in the Pentagon, the State Department, and the White House. The crux of his story is that a promising negotiation was tragically cut short, just as it was about to bear fruit, when the key negotiator, a Marine colonel, was “PNG’d”—declared persona non grata—by Washington and denied entry to Jordan. Not long after that, Gaood died suddenly, of a heart ailment, at the age of forty-four (according to Perry, he was so beloved that his wake had to be held in a soccer stadium), putting an end to any possibility of further talks. It’s startling to read about American military commanders in the field taking on a freelance diplomatic mission of this magnitude, and to imagine that there was a businessman in Amman who, on the right terms, could have snapped his fingers and ended what we back home thought of as pervasive, wild-eyed jihad.

What dominates the writing of experts about terrorism, however, is a more fine-grained idea of terrorists’ motives—at the level of ethnic group, tribe, village, and even individual calculation. Pape thinks of terrorists as being motivated by policy and strategic concerns; Cronin, of the National War College, shares Pape’s view that most terrorists are, essentially, terroirists—people who want control of land—but she is also attuned to their narrower, more local considerations. The odds are against them, because of the natural forces of entropy and their lack of access to ordinary military power and other resources, but, if they do succeed, they can be counted upon to try to ascend the ladder of legitimacy, first to insurgency, then to some kind of governing status. (Examples of that ultimate kind of success would be the Irgun and the Stern Gang, in Israel, Sinn Fein and the Provisional I.R.A., in Northern Ireland, and the Palestine Liberation Organization, in the West Bank and Gaza.)

Cronin goes through an elaborate menu of techniques for hastening the end of a terrorist campaign. None of them rise to the level of major policy, let alone a war on terror; in general, the smaller their scope the more effective Cronin finds them to be. She believes, for instance, that jailing the celebrated head of a terrorist organization is a more effective countermeasure than killing him. (Abimael Guzmán, the head of the Shining Path, in Peru, was, after his capture in 1992, “displayed in a cage, in a striped uniform, recanting and asking his followers to lay down their arms.” That took the wind out of the Shining Path’s sails. A surprise ambush that martyred him might not have.) Negotiating with terrorists—a practice usually forsworn, often done—can work in the long term, Cronin says, not because it is likely to produce a peace treaty but because it enables a state to gain intelligence about its opponents, exploit differences and hive off factions, and stall while time works its erosive wonders.

Cronin offers a confident prescription, based on her small-bore approach to terrorism, for defeating the apparently intractable Al Qaeda. The idea is to take advantage of the group’s highly decentralized structure by working to alienate its far-flung component parts, getting them to see their local interests as being at odds with Al Qaeda’s global ones. “Bin Laden and Zawahiri have focused on exploiting and displacing the local concerns of the Chechens, the Uighurs, the Islamic Movement of Uzbekistan, the Salafist Group for Call and Combat in Algeria, and many others, and sought to replace them with an international agenda,” Cronin writes. The United States should now try to “sever the connection between Islamism and individualized local contexts for political violence, and then address them separately.” It should work with these local groups, not in an effort to convert them to democracy and love of America but in order to pry them away, one by one, from Al Qaeda. (“Calling the al-Qaeda movement ‘jihadi international,’ as the Israeli intelligence services do,” she writes, “encourages a grouping together of disparate threats that undermines our best counterterrorism. It is exactly the mistake we made when we lumped the Chinese and the Soviets together in the 1950s and early 1960s, calling them ‘international Communists.’ ”)

Eli Berman, an economist who has done field work among ultra-orthodox religious groups in Israel, is even more granular in his view of what terrorists want: he stresses the social services that terror and insurgent groups provide to their members. Berman’s book is an extended application to terrorism of an influential 1994 article by the economist Laurence Iannaccone, called “Why Strict Churches Are Strong.” Trying to answer the question of why religious denominations that impose onerous rules and demand large sacrifices of their members seem to thrive better than those which do not, Iannaccone surmised that strict religions function as economic clubs. They appeal to recruits in part because they are able to offer very high levels of benefits—not just spiritual ones but real services—and this involves high “defection constraints.” In denominations where it’s easy for individual members to opt out of an obligation, it is impossible to maintain such benefits. Among the religious groups Iannaccone has written about, impediments to defection can be emotionally painful, such as expulsion or the promise of eternal damnation; in many terrorist groups, the defection constraints reflect less abstract considerations: this-worldly torture, maiming, and murder.

Berman’s main examples are Hamas, Hezbollah, Moqtada al-Sadr’s Mahdi Army, in Iraq, and the Taliban, whom Berman calls “some of the most accomplished rebels of modern times.” All these organizations, he points out, are effective providers of services in places where there is dire need of them. Their members are also subject to high defection constraints, because their education and their location don’t put them in the way of a lot of opportunity and because they know they will be treated brutally if they do defect.

Like most other terrorism experts, Berman sees no crevasse between insurgents and terrorists. Instead, he considers them to be members of a single category he calls “rebels,” who use a variety of techniques, depending on the circumstances. Suicide bombing represents merely one end of the spectrum; its use is an indication not of the fanaticism or desperation of the individual bomber (most suicide bombers—recall Muhammad Atta’s professional-class background—are not miserably poor and alienated adolescent males) but of the supremely high cohesion of the group. Suicide bombing, Berman notes, increases when the terrorist group begins to encounter hard targets, like American military bases, that are impervious to everything else. The Taliban used traditional guerrilla-warfare techniques when they fought the Northern Alliance in the mountains. When their enemies became Americans and other Westerners operating from protected positions and with advanced equipment, the Taliban were more likely to resort to suicide bombing. How else could a small group make a big impact?

The idea of approaching terrorists as rational actors and defeating them by a cool recalibration of their incentives extends beyond the academic realm. Its most influential published expression is General David Petraeus’s 2006 manual “Counterinsurgency.” Written in dry management-ese, punctuated by charts and tables, the manual stands as a rebuke of the excesses of Bush’s global war on terror.

“Soldiers and Marines are expected to be nation builders as well as warriors,” the introduction to the manual declares. “They must be prepared to help reestablish institutions and local security forces and assist in rebuilding infrastructure and basic services. They must be able to facilitate establishing local governance and the rule of law.” The manual’s most famous formulation is “clear-hold-build,” and its heaviest emphasis is on the third of those projects; the counterinsurgent comes across a bit like a tough but kindhearted nineteen-fifties cop, walking a beat, except that he does more multitasking. He collects garbage, digs wells, starts schools and youth clubs, does media relations, improves the business climate. What he doesn’t do is torture, kill in revenge, or overreact. He’s Gandhi in I.E.D.-proof armor.

Petraeus has clearly absorbed the theory that terrorist and insurgent groups are sustained by their provision of social services. Great swaths of the manual are devoted to elaborating ways in which counterinsurgents must compete for people’s loyalty by providing better services in the villages and tribal encampments of the deep-rural Middle East. It’s hard to think of a service that the manual doesn’t suggest, except maybe yoga classes. And, like Berman, the manual is skeptical about the utility, in fighting terrorism, of big ideas about morality, policy, or even military operations. Here’s a representative passage: