From the bombers of 9/11 to the Tsarnaev brothers, everyone asks the question: why? Why would these men kill? Why would these men aim for such destruction? We know there is no one path to radicalization. The reasons why someone picks up a gun or blows themselves up are ineluctably personal, born variously of grievance and frustration; religious piety or the desire for systemic socio-economic change; irredentist conviction or commitment to revolution. And yet, though there is no universal terrorist personality, nor has a single, broadly applicable profile ever been produced, there are things we do know. Terrorists are generally motivated by a profound sense of (albeit, misguided) altruism; deep feelings of self-defense; and, if they are religiously observant or devout, an abiding, even unswerving, commitment to their faith and the conviction that their violence is not only theologically justified, but divinely commanded.

All terrorists fundamentally see themselves as altruists: incontestably believing that they are serving a “good” cause designed to achieve a greater good for a wider constituency—whether real or imagined—which the terrorist and his organization or cell purport to represent. Indeed, it is precisely this sense of self-righteous commitment and self-sacrifice that that draws people into terrorist groups. It all helps them justify the violence they commit. It gives them collective meaning. It gives them cumulative power. The terrorist virtually always sees himself as a reluctant warrior: cast perpetually on the defensive and forced to take up arms to protect himself and his community. They see themselves as driven by desperation——and lacking any viable alternative—to violence against a repressive state, a predatory rival ethnic or nationalist group, or an unresponsive international order.

Religion only serves as one more justification—particularly in the case of suicide terrorism. Theological arguments are invoked both by the organizations responsible for the attacks and by the communities from which the terrorists are recruited. In the case of Muslims, although the Quran forbids both suicide and the infliction of wanton violence, pronouncements have also been made by radical Muslim clerics, and in some instances have been promulgated as fatwas (Islamic religious edicts), affirming the legitimacy of violence in defense of defenseless peoples and to resist the invasion of Muslim lands. Among the most prominent was the declaration by the Ayatollah Khomeini who once declared (in the context of the Shi’a interpretation of Islam) that he knew of no command “more binding to the Muslim than the command to sacrifice life and property to defend and bolster Islam.” Radical Islamist terrorist movements have thus created a recruitment and support mechanism of compelling theological incentives that sustains their violent campaigns.

But individuals will always be attracted to violence in different ways. Just look at the people who have gravitated towards terrorism in the United States in recent years. We have seen terrorists of South Asian and North as well as East African descent as well as those hailing both from the Middle East and Caribbean.

Now we have the Tsarnaev brothers products of centuries-long conflict between Russia and Chechnya. We have seen life-long devout Muslims as well as recent converts—including one Philadelphia suburban housewife who touted her petite stature and blonde hair and blue eyes as being so atypical of the stereotypical terrorist so as to defy any efforts at profiling. Radicalized over the Internet, she sought to use her self-described ability to avoid detection to assassinate a Swedish artist who drew a cartoon of the Prophet Muhammad. They come from every walk of life, from marginalized people working in menial jobs, some with long criminal records or histories of juvenile delinquency, to people from solidly middle and upper-middle class backgrounds with university and perhaps even graduate degrees and prior passions for cars, sports, rock music and other completely secular, material interests.

The spectrum of British jihadis over the past decade illustrates the panoply of individuals attracted to terrorism. Richard Reid, the so-called “shoe bomber,” who attempted to blow up an American Airlines flight en route from Paris to Miami in December 2001 was a career criminal who dropped out of high school and converted to Islam while in prison before he was recruited to al Qaeda. By comparison, Omar Khyam, the mastermind behind a 2004 bombing plot of a shopping mall outside London, was the son of a wealthy businessman and grew up in a comfortable, upper-middle-class environment. Similarly, Ahmed Omar Saeed Sheikh, the al Qaeda operative responsible for the kidnapping of Wall Street Journal reporter Daniel Pearl in 2002, attended an exclusive (e.g., expensive) private school. He later was admitted to the world-renowned London School of Economics (LSE), where he studied applied mathematics statistical theory, economics, and social psychological. Described as “handsome, tall and muscular, very bright and charming,” his parents expected he would be knighted some day and not languishing in prison awaiting execution for his role in Pearl’s execution.

And, as we have also seen, relationships formed at work, at school, on sports teams, and other recreational and religious activities as well as over the Internet can prey upon the already susceptible. In some instances, first generation sons and daughters of immigrants embrace an interpretation of their religion and heritage that is more political, more extreme and more austere—and thereby demands greater personal sacrifices—than that practiced by their parents. The violence inflicted on Muslims in general and Muslim women and children especially around the world have been cited by many homegrown terrorists as a salient motivating factor in their politicization and radicalization and may explain why the American invasions of Iraq and Afghanistan were cited by Dzhogar Tsarev as the reasons behind his and his older brother’s bombing of the Boston Marathon.

Indeed, the common element in the radicalization process reflects these individuals’ deep commitment to their faith—often recently re-discovered; their admiration of terrorist movements or leading terrorist figures who they see as having struck a cathartic blow for their creed’s enemies wherever they are and whomever they might be; hatred of their adopted homes, especially if in the United States and the West; and, a profoundly shared sense of alienation from their host countries.

At the start of the war on terrorism a dozen years ago the enemy was clear and plainly in sight. It was a large terrorist organization, situated mostly in one geographic location, and it was led by an identifiable leader. Today, when the borders between domestic and international terrorism have blurred, when our adversaries are not only identifiable organizations but enigmatic individuals, a complete re-thinking of our counterterrorism policies and architecture is needed. We built an effective defense against the previous threat. Our challenge today is to develop new defenses against this more amorphous, diffuse and individualized one.