Prior to September 11, 2001, few Americans registered serious concern about terrorism in the United States. The attacks of 15 years ago were, and remain, the deadliest terrorist attacks in history. And yet terrorism was a deadly phenomenon around the world for decades before those attacks, and was the subject of study among a small community of researchers as early as the 1960s. What’s truly different about the terrorism of the post-9/11 era, and what’s been consistent over time? And why does the problem still seem so difficult to manage? Below, three of the scholars who helped define the modern field of terrorism research reflect on what’s been learned, what’s been forgotten, and what still isn’t known about why terrorists attack.

Brian Michael Jenkins: The bookshelf on terrorism understandably expanded rapidly after the 9/11 attacks brought the issue so forcefully to the attention of the American public. Much of this literature reflects the commercial desire of publishers to exploit intense public interest. A lot of the entries fall into the category of lurid sensationalism and forecast imminent doom (“Al-Qaeda already has nukes in New York”) offer conspiracy theories (“What the government won’t tell you”), or feed partisan agendas. As such, it informs us more about the country’s state of anxiety than it does about terrorism. Memoirs of any former government official vaguely connected with counterterrorism also found publishers. But some of the new terrorism literature reflected excellent investigative journalism and offered insights about the nature of the adversary, a lot of it focusing on the specific issues faced by the United States as it went to war. Without 9/11 and the “global war on terror,” it is doubtful that there would so many histories of Afghanistan or Pakistan. We had to learn a great deal more about the specific terrorists we were up against.

Certain blank spots remain unfilled. How terrorists think about what they decide to do is one of them. During World War II, American generals studied the writings of their German counterparts. Remember the famous line from the movie Patton, when George C. Scott triumphantly shouts out, “Rommel, you magnificent bastard, I read your book.” During the Cold War, a great deal of scholarly research was devoted to understanding Soviet behavior. Terrorists were early on dismissed as evil, mad dogs—no further inquiry was necessary. The methods of interrogation, closed prosecutions by military tribunals, and the rules of classification have kept information out of the public domain. It would be useful to know the arguments that took place in al-Qaeda—surely there were some—about the intentions and likely consequences of the 9/11 attacks as its leaders discussed their plans.

The determination of today's terrorists to kill in quantity, and difficulties in predicting terrorist attacks and protecting all possible targets, or in striking back against attackers who often die in the attacks, have pushed the authorities in the direction of pre-emptive intervention. This includes efforts to counter radicalization and recruitment to terrorism. Who will become a terrorist remains difficult to predict. Efforts in the 1970s to understand why terrorists became terrorists produced little that was operationally useful. Terrorists were not crazy in the clinical sense. There were no discernible personality attributes beyond seeing the world in the black-and-white, us-versus-them mindset of all true believers. There are, however, many such people. We know only about those who become terrorists. Predicting dangerousness remains difficult. I am not persuaded that our current models of how people become radicalized and are recruited, or recruit themselves, to terrorism are accurate.

The fundamental philosophical questions remain: How much security can a government be expected to provide its citizens? What is the obligation of a nation to its citizens if they are held hostage abroad? Do targeted killings differ from assassinations? And are such killings a preferable, even more moral alternative, to less discriminate military operations? How do liberal democracies effectively deal with violent adversaries capable of great violence and remain democracies? Or will perpetual war incrementally push us toward tyranny.

Bruce Hoffman: With respect to the vast literature that has emerged on terrorism in the decade and a half since the 9/11 attacks, I have often marveled at how well many of the seminal works on the subject written over 40 years ago by both of you, Walter Laqueur, the late Paul Wilkinson, and others have stood the test of time. To this day, the syllabi for my terrorism courses contain as many (and perhaps even more) works that date back to that era than contemporary works.

This is not meant to imply that there have not been some enormously useful and important contributions to the literature in recent years, but rather to bemoan a proclivity towards amnesia, ignorance and a reinventing the wheel—often accompanied by the application of arcane quantitative methodologies—to aspects of terrorism that has added little to the corpus of work often published decades ago.

As the editor of a scholarly journal focusing on conflict and terrorism, I am continually surprised by how many submissions treat terrorism as something that began on 9/11 and whose authors seem to be unaware of the wealth of research and literature which predated that watershed event. Similarly, the fashion in terrorism studies today is highly methodological treatments replete with voluminous accompanying tables, figures, graphs, and statistical interpretations. While impressive in a purely technical sense, I often find little in them that is either new or genuinely advances the field or improves our understanding of the phenomenon.

There is also an understandable “herding” aspect to contemporary terrorism research around whatever the issue or threat du jour happens to be. It was suicide terrorism a decade ago, radicalization more recently, and “lone wolves” today. Less common in my experience are submissions that focus less on what is topical and in the news and more on what is unique, unusual, trendsetting, anticipatory, or novel. The literature on terrorism that authoritatively draws on historical comparisons, contemporary analogues, new theoretical interpretations, or truly innovative approaches seems to have become far less prevalent in recent years.

Your point, Brian, about the blank spots that remain unfilled in the study of terrorism is absolutely correct. These are timeless questions that go to the heart of our efforts to understand terrorists and terrorism and, as you and Martha well know, are by no means new.

For decades, scholars and analysts have searched for this holy grail of terrorism studies that would unlock all the mysteries about why individuals embrace violence in this manner. I am reminded here of the massive study undertaken by the West German government in the early 1980s in hopes of divining such an answer. The result was a multi-volume, meticulously detailed compendium of years of research that, as I recall, identified no single reason or universal explanation of why someone becomes a terrorist (or, in the contemporary vernacular, how one is radicalized). Much as we still lack an understanding of why persons become terrorists, we also still lack an understanding of how governments can best and most effectively respond to this menace.

The West Germans’ failure to answer this provides a revealing window on the limits of contemporary research into the radicalization process that consumes so much scholarly and popular attention today. The West Germans of course had the luxury of focusing on a very discrete and homogeneous demographic. The Red Army Faction (RAF, or Baader-Meinhof Gang) and Revolutionary Cells, for instance, each had only a few dozen active members at most. They all came from the same country and had roughly similar backgrounds and socio-economic characteristics. Yet, even this concerted effort surveying and analyzing a small pool of violent individuals yielded no meaningfully broad explanations or useful generalizations. Compare that to today, when terrorist movements like ISIS have recruited tens of thousands of followers and fighters from some 120 different countries, and the challenges and limits of discerning how persons are radicalized and why they become terrorists become obvious.

Finally, to the fundamental questions, Brian, that you elucidate in your final paragraph, I would add that much as we still lack an understanding of why persons become terrorists, we also still lack an understanding of how governments can best and most effectively respond to this menace.

In this respect, the fact that terrorism is a strategy of provocation is often forgotten or neglected. Terrorists have arguably always attempted to provoke governments to react emotionally and precipitously to threats rather than respond in a sober and rational manner. Many critics charge that this is precisely what happened in the aftermath of the 9/11 attacks with the declaration of an expansive “war on terror.” Yet, governments seem to have continually fallen into this trap—with the predictable result that we remain enmeshed in spiraling cycles of violence and campaigns with no end apparently in sight.

Hand-in-glove with this is a failure to understand that terrorism is also a strategy of attrition. It is designed not only to wear down the terrorists’ government opponents and undermine the morale of both the authorities themselves and the citizens they either represent or are charged with protecting, but also over time to create deep fissures in national polities, to undermine public confidence in elected leaders, to foment paranoia and xenophobia, and thus cause liberal societies to adopt increasingly illiberal means to enhance security and supposedly better protect and defend against this threat. One just has to look at the divisive debates and political campaigns today in the United States, Germany, and elsewhere to see the corrosive effects that this terrorist strategy is having on our societies and political systems.

In this respect, the one thing that has changed today from the past is that the stakes are arguably bigger and more consequential than ever before. But that is a matter perhaps best taken up in the next exchange.

Martha Crenshaw: The tendency to oversimplify what is an extraordinarily complex phenomenon is still very much with us, especially with policymakers. People want to see terrorism through one prism. Or to see it in binary terms: It is either the work of evil fanatics or of misguided youth. The messy reality is hard to deal with. For many years researchers have been trying to explain that there is no single “terrorist profile,” but such a profile still seems to be the holy grail. Bruce is right about the West German study (five volumes); another interesting detail is that, of the over 200 leftist terrorists in jail in West Germany when the study was conducted, only a handful would agree to be interviewed.

This points to a general impediment to theory building: Terrorism is actually rare except in certain concentrated spaces like Northern Ireland in the past, and perhaps Iraq now. Also, there aren’t many terrorists (or jihadists in contemporary idiom). There are many people who fit whatever profile can be drawn up, and a tiny number resort to violence. And within that small subset there can still be immense variation in motivation.

It is almost hard to remember what it was like before 9/11, and I think of our current students who were small children and have no basis for comparison of then and now. So much changed in such a short time. And the big question is of course “are we safer?” But seen in finer grain, is the massive counterterrorism bureaucracy the U.S. has built since 9/11 effective, even modestly so? I am reminded of the fact that many of the recommendations of the 9/11 Commission did not address the issues they identified as the problem. So we have solutions that don't even appear to be in search of problems—as soon as the threat is defined as “terrorism” every apparent solution in sight is applied. Not to be too cynical, of course! But we haven't found good ways to measure the effectiveness of counterterrorism measures.

Like both of you, I am not a statistical researcher, and although I see the value of large databases (and have been involved with the Global Terrorism Database at University of Maryland from the beginning), I fear that researchers who have not worked through the arduous process of collecting data/evidence may take for granted the accuracy and comprehensiveness of datasets collected and classified by others. Analyses of aggregate data need to be aware of the limitations.

I agree with Bruce that terrorism is often intended to provoke (as jihadist ideologues will tell us themselves, e.g., through the book The Management of Savagery, which appeared online in 2004), and also to erode resolve and democratic values. Paul Wilkinson always stressed the threat of terrorism to liberal democracy. It can also be meant simply to punish. In that sense, it is no longer coercion. It is more akin to vengeance, but perhaps not exactly the same thing.

As Brian says, researchers could spend more time and effort trying to understand what the decision makers on the terrorist side are thinking. With regard to provocation, a subject I don't see much about is how terrorism provokes military intervention or escalation of existing intervention in civil wars—we've seen arguments that military intervention provokes terrorism, but has anyone argued that it's the reverse? Transnational terrorism that targets outside states draws them into local conflicts.

Along these lines, I often think that our current counterterrorism focus on “countering the narrative” neglects to consider how our actions affirm and reinforce the jihadist narrative. Perhaps I am too critical here, but it seems to me that American leaders and opinion makers in particular think of their/the enemy's “narrative” as entirely self-generated. We see that it is emotionally powerful, but we don't see why.

This problem is related to a tendency to spotlight the means of transmission of propaganda (social media now, tapes and cassettes of sermons years ago, radio even!) rather than the content of the message.

Brian Michael Jenkins: There is a notion that on 9/11, the world changed, and in many respects it did. But that has become a basis for discarding what happened and what was learned before. Bruce is right. We reinvent the wheel.

Bruce and Martha have both mentioned the famous study of German terrorists that with Teutonic thoroughness examined every aspect of their lives only to conclude that there was no distinguishing profile. Four decades later, we are launching efforts to counter radicalization. Some of these are local efforts to improve communications with communities believed susceptible to radicalization and recruitment. Some are efforts to enlist respected members of these communities to speak out against ideologies that promote violence. Some are broader efforts to counter incitement via social media. Some are efforts to identify individuals at risk in order to dissuade them from taking a destructive and self-destructive path. Judging by the British intervention program called PREVENT, this requires an intense and long-term effort. Dissuading individuals from becoming terrorists is, of course, preferable to shooting them later or putting them in prison. But we are still working in the dark. As Martha points out, there are a lot of people with radical ideas—which is not a crime—but only a handful of those holding extreme views will become terrorists. Dangerousness is difficult to predict.

We also know that radicalization is not a straight path. One individual may gradually radicalize over a long period, becoming more extreme and ultimately turning violent. Another may proceed directly toward violence, donning an ideological cloak or claiming affiliation with some group just before they act. Still others appear to head down the path toward violence, then back off. Much depends on what else is going on in their lives. Personal crisis may propel an individual toward violence. A good job or a newfound love may change one's course. How do you predict that?

Absent a profile we take a more wholesale approach, educating and enlisting entire communities where it is believed radicalization and recruitment are occurring or might occur. That appears to have worked in reducing the recruitment of young Somali men out of a particularly affected community in Minnesota where there was a recruiting network. Will it work elsewhere?

As authorities push upstream to intervene before terrorist-related crimes are committed, we have to be careful to avoid patrolling ideologies. What seems to be a sensible preventive measure can slide into policing beliefs.

Bruce mentions the objective of terrorists to provoke overreaction, and Martha raises concerns about the threat terrorism poses to liberal democracies. While American democracy remains strong, we have witnessed extraordinary changes over the past 15 years. It would be incorrect to say that civil liberties have been savaged, but we have laid the foundation for what, under a less benign government or a more frightened populace, could become a more oppressive state.

As the “old-timers” of the terrorism research community, we cannot claim to be smarter than anyone else, but we have been around for a long time. Historical perspective is valuable. It does not mean we have the answers. I was invited to testify before Congress for the first time in 1974. I was flattered and nervous. I carefully crafted my written testimony, rehearsed my oral testimony—my colleagues grilled me with questions that might arise. Had the members of the congressional committee asked, “How many machine guns does the IRA have?” I was ready. As I recall, one of the first questions a congressman asked me was, “Mr. Jenkins, how can we end terrorism?” I think I mumbled on for several minutes, pointing out that there were some things beyond even the august legislative powers of the United States Congress. If I were asked that question today, I am not sure I could give a better answer.

We have survived more than four decades of terrorism. People are shocked when I point out that during the 1970s, the United States survived 50 to 60 terrorist bombings a year. Imagine that volume of terrorist activity today. Had I, right after 9/11, forecast that 15 years later, we would still be engaged in military operations against terrorists in various parts of the world, my audience no doubt would have regarded that as dismaying, even though the analysts at the time reckoned that this would be a very long struggle. They would have been relieved to hear that in the following 15 years, fewer than a hundred people would be killed in the United States by jihadist terrorists. All deaths are tragic, but we have suffered far less at home than we feared in the immediate shadow of 9/11.

We need to take terrorism seriously, but we ought not to inflate the terrorist threat. In the interest of promoting action abroad and preparedness at home, our own officials often amplify terrorist threats. Even some generals, who should know better, describe the current contest with the Islamic State as World War III. It is not. True, our military efforts in Afghanistan and Iraq have been costly—10,000 Americans killed, 50,000 wounded. In World War II, more than 400,000 died. And in that conflict, the United States, with a population of 130 million people, put 16 million in uniform, about 12 percent of the entire population. The equivalent today would be something like 38 million. The entire nation then was united in the war effort. Today, only the families of those in uniform are affected. There is no national unity, no sense of shared sacrifice, just the constant whine, "Are we safer now?"

Terrorism involves not just the terrorists and their counterterrorist adversaries. Terrorism is aimed at the people watching. It is intended to produce fear, which will, in turn, cause us to exaggerate the threat. And it often works. Research focuses on the terrorist threat and the countermeasures. We devote less attention to the reactions of the audience. We don't want to look at us.

Bruce Hoffman: The oversimplification that Martha refers to is of course completely encapsulated by the question posed to Brian more than 40 years ago by the congressman who asked, “How do we end terrorism?” A truthful answer today would be that, despite all the books, studies, analyses, simulations and exercises, we still don’t know.

We do know how to mitigate the threat through physical security and other preventative and preemptive techniques. And, we know how to kinetically weaken terrorist groups. But as you both correctly observe, we have had far less success in countering their narrative much less breaking the cycle of recruitment and regeneration that has now enabled successive generations and branches of al-Qaeda to emerge—and which also produced an even more sanguinary off-shoot in ISIS.

We have thus experienced many tactical successes, but strategic victory against our terrorist adversaries remains elusive.

The fundamental problem is that they have proven more capable of adjustment and adaptation than we have. Terrorist groups seem to learn faster and better than in the past because they have to in order to survive. They have certainly shown themselves to be both more adept and far quicker in harnessing the power and potentiality of social media and other 21st-century digital communications platforms than their state opponents. They are also, frustratingly, now able to communicate more securely amongst themselves than ever before simply by utilizing readily available commercial apps downloaded from the internet.

The terrorists’ skill at exploiting the array of 21st-century communications technologies, compared with the pre-digital limitations that existed only a decade ago, must surely factor into the astonishing longevity of many contemporary terrorist groups. This month, for instance, marks the 15th anniversary of the 9/11 attacks, but we forget that just a few weeks ago al-Qaeda “celebrated” its 28th year in existence. Hezbollah, to cite another example, is 34 years old; Lashkar-e-Taiba is 30; and Hamas is only one year younger. The longevity of these preeminent terrorist groups is testament perhaps to the intractable fixture of conflict in the 21st century that terrorism has become—and likely will remain.

Accordingly, one has to wonder whether from the terrorists’ perspective they think they are losing. The threat posed by ISIS’s caliphate may soon prove to have been a flash in the pan, but the fact that al-Qaeda has survived the greatest worldwide onslaught ever directed against a terrorist group by the most technologically advanced military in the history of mankind underscores how much more challenging counterterrorism is today compared with the 20th century—when many left-wing groups were small enough to be crushed by police action or simply outlived their relevance.

Brian Michael Jenkins: Prior to 9/11 and the declaration of the “global war on terror,” the government used the phrase “combatting terrorism.” War, in modern American military tradition, implies a finite undertaking—a clear beginning and end, while “combatting terrorism” implies an enduring task. In war, we seek victory. In combatting crime, we have no expectation of an ultimate police victory over all criminals and the end of crime. Instead, we expect the authorities to keep crime under control—that is, within limits society can tolerate. As Bruce points out, we can weaken terrorist groups, create a hostile environment for them, and take other measures to mitigate the threat, but can we realistically expect to achieve a final victory over terrorism?

The mobilization that the term “war” implied was a useful conceptual framework right after 9/11. It also signaled that the United States would not simply strike back and then wait for the next terrorist attack, as it had in the past, but that this was going to be a continuing campaign to bring the perpetrators of the 9/11 attacks to justice, to disperse and destroy their organization—at least to the point that it was no longer capable of mounting another 9/11-scale attack. That was a more limited undertaking, but one that was going to require military force as well as intelligence work, diplomacy, law enforcement and all of the other instruments we could muster. It was anticipated that this would be a very long effort and we haven't succeeded yet. America's terrorist foes have been nimble and resilient and tenacious, as Bruce indicates, but they have also benefitted from dysfunctional governments, waves of protest, and ongoing conflicts across North Africa, the Middle East, and South Asia, and they have been able to fasten themselves on to local grievances and struggles, convincing their local allies and persuading the world that this is all part of a vast global jihad. What it is is marketing genius.

The terrorists have a second advantage. It is inherently less difficult to exploit anger and fuel violence than it is to bring or restore order. In continuing efforts to combat terrorism, the United States must be cautious not to assume the mission of removing every tyrant, fixing every failed state, eliminating every ungoverned space—in other words, reconstructing and policing the world. As we reflect upon 9/11, one poll asked whether the world is now a safer place. Is that America's burden?

Quick sidebar here: Over the long run, despite reported increases in terrorism, the declining number of armed conflicts in the world with fewer and fewer casualties from war suggest that, statistically, the world is, in fact, a safer place. But few think so.

With that, I'm off to the Middle East. Thanks and best wishes to all.

Martha Crenshaw: Bon voyage, Brian. We are getting security warnings here in Paris. Avoid places where tourists congregate, they say. In Paris! There are tourists everywhere.

Brian said years ago that terrorism was theater, and he is still right. The audience is the key to its effectiveness. I don't know that governments always keep that in mind. At least in the U.S. our counterterrorism “strategies” are usually framed at such a high level of generality that it is hard to integrate strategy and tactics. I don't think we've done a good job of linking the war on terrorism or AQ/ISIS to “countering violent extremism” (CVE), for example; it has not been possible to switch from war to CVE, but how do we combine them?

Why groups like AQ and others have proved so persistent and adaptable is a big question. Bruce points to their mastery of communications, linking them to their audiences in ways that weren't possible 15 years ago. If we stick to the theater metaphor, then we see that opportunities in failed and failing states, discredited authoritarianism, ethnic conflicts, and other local dysfunctions have provided a series of stages for their efforts. We, meaning the U.S. or the West, can't close off these opportunities, certainly not in the short run and probably not in the long run. We are also forced into compromises of our values in order to maintain order and create some sort of bulwark of stability in the short term. It’s very frustrating.

I agree that terrorists’ conception of "success" or "victory" is different from ours (not that we have a clear conception on our end). ISIS ideologues are preparing their audiences for the long haul. Losing the caliphate? Only a temporary setback, predicted by history.

Threat exaggeration has been encouraged by the politicization of the issue. It has become difficult for anyone to say that the threat is manageable, even though governments try to encourage “resilience.” The current presidential campaign is a good example.

Bruce Hoffman: Martha nails it when she writes that our enemies' conception of success and victory are vastly different than our own, and that losing the caliphate is, in their eyes, a mere tactical reversal. When a struggle is divinely ordained—as both al-Qaeda and ISIS claim theirs to be—all temporal setbacks or defeats are inconsequential and mere speed bumps on the highway to triumph: challenges deliberately put in the terrorists’ path to test their fealty and ultimate commitment to the cause and the group that they serve.

In that respect, I wonder if bin Laden were alive today, how he would see things on this 15th anniversary of the September 11th attacks? Unfortunately, I think he would be rather pleased for several reasons.

First, bin Laden once remarked in a 1998 interview that he was not afraid of death and welcomed martyrdom. This was because he was confident that his death would "produce thousands of Osamas." Indeed, with the proliferation of tens of thousands of foreign fighters now engaged in combat in Afghanistan, Iraq, Syria, Libya, Yemen, and Mali, his prediction has arguably (and tragically) come true.

Second, exactly six years ago, bin Laden called on his followers to carry out “Mumbai-style attacks” (in reference to the bloody, simultaneous terrorist strikes which convulsed that Indian city in November 2008) in European cities. There were no takers, since none of his acolytes were then able to do so. In the past year, however, we have seen the horrific attacks in Paris and then in Brussels that conform to this model. Admittedly, they were perpetrated by ISIS and not al-Qaeda or an al-Qaeda affiliate, but it is often forgotten that ISIS, notwithstanding its expulsion from al Qaeda in 2014, still reveres bin Laden and regards itself as the terrorist leader's most faithful progeny.

Third, we know from the documents seized by the U.S. Navy SEALs at bin Laden's Abbottabad lair in May 2011 that he sought to rebrand al-Qaeda and endow it with a moniker that also reflected its political aims and was less pejorative than the movement's original name. With the emergence of groups like Jabhat al-Nusra (now Jabhat Fatah al-Sham); Ahrar al-Sham; the variety of entities in Libya, Yemen, Tunisia and elsewhere calling themselves one form of "Ansar" or another; and many other violent Salafist-jihadi groups who ascribe to al-Qaeda's ideology, and often follow the orders of al-Qaeda leader Ayman al-Zawahiri, but deliberately eschew the al-Qaeda designation, bin Laden's hopes in this respect have been realized as well. Even worse, these "jabhats" and "ansars" are often regarded in the regions in which they fight as more "moderate" and hence acceptable than ISIS. This is a pernicious and lamentable development, indeed.

Finally, in 2005 bin Laden was presented with a seven-stage strategy to reverse al-Qaeda's then-declining fortunes and launch it on a trajectory to inevitable victory. Today, the al-Qaeda movement is arguably right on schedule in having fulfilled the first four stages of this epic battle plan. Regardless of whether it will ever be realized or completed, for the time being this successful progression feeds al-Qaeda's narrative, polishes their image to new and greater luster, and enabled the movement founded by bin Laden nearly three decades ago to carry on their struggle by continuing to attract recruits and support (financial and otherwise)—thus ensuring al-Qaeda's survival, at least for the immediate future.