If so many young, would-be terrorists are motivated to fight by the U.S. presence in Iraq, is a U.S. withdrawal enough to ease their anger? How would American troops leaving Iraq change the equation?

Watching images of Americans killing Muslims inflames Muslim moral outrage worldwide. Such images need to be eliminated through having a much smaller footprint in Iraq. Just like American troops in Japan, it is not the presence of troops per se that is causing this outrage, but the perceived moral violations they cause. So, in Japan, each case of U.S. military personnel accused of raping young Japanese women triggers large demonstrations calling for withdrawal of the U.S. presence there. It’s a similar process for Iraq, but those images are broadcasted to the Muslim world through satellite channels, fueling Muslims’ anger everywhere. Eradicating such images will diminish this sense of outrage and diminish the desire of Muslims to strike back against the United States.

The examples of young terrorists you cite in your article were mostly living in the West, places like Canada and Western Europe. Is that where we should look for the next attack to come from—not, say, Waziristan in Pakistan or Iraq?

The danger comes from the West. And like with the attacks of September 11, the threat to the continental United States comes from Europe. Muslims who are living in the West would be able to settle and operate in the United States. The 15 Saudi perpetrators involved in the September 11 plot had to be closely hand-held by the leaders of the plot, who had all lived extensively in the West. People from Waziristan or Iraq would have trouble operating in the United States undetected.

I believe the military is certainly better equipped and trained to capture or kill terrorists than law enforcement. Doesn’t “demilitarizing” the struggle against terrorism, as you suggest, amount to pulling out of Iraq and Afghanistan and letting the terrorists win? How can law enforcement be nearly as effective?

This is a common misconception. Terrorism is never a military problem. People confuse it with an insurgency or even a civil war because the atrocities committed in these conflicts employ terrorist tactics. Terrorists in the West hide in the local population. This is a law enforcement problem because the police know the community, whereas the military rarely does. On the other hand, when terrorists are able to congregate in sanctuaries, as they did in Afghanistan before late 2001, then the destruction of these sanctuaries is a military mission. The use of military force lumps all our potential enemies together and unifies them needlessly. The appropriate military mission should be limited to sanctuary denial.

You write that young people often find violent images online that lead them to want to commit terrorist acts. What key images, phrases, and motifs are most often aimed at these young people to get them to join these new terrorist networks? And which of these are the most effective?

What triggers moral outrage among Muslims, as it would for any other person, is the image of a moral violation against a person with whom they empathize. These images are projected by the media, like Al Jazeera. They are part of reporting of events, and as such they are not directed at people. They are just the news. The rage they cause helps fuel the desire to get back at the violators on the screen.

However, al Qaeda propagandists project other images that try to mobilize young men to join the movement. These images are about the glory of being a jihadist. These are images of the rigors of training, the rituals of jihadists praising their fight, and especially the images of successful bombing attempts. Young men are chasing fantasies of glory and thrill, and these al Qaeda images play to these desires.

There’s no doubt that terrorists use the Internet to communicate with one another. You call radical Web forums the “invisible hand” organizing terrorist activities today. But given the nature of the Internet, what can possibly be done to monitor and stop such communications?

This is an excellent question that has implications far beyond the issue of terrorism. Online communication is becoming a foundation for our society. Whatever is done in influencing such communication must be a societal decision based on a broad debate. I would not want the government to decide unilaterally what should be done. I would not rush into monitoring or eliminating even distasteful communication without a healthy social debate about them.

Studies by scholars such as Alan Krueger, Robert Pape, and yourself have shown definitively over the years that poverty, lack of education, and religious fanaticism do not make a terrorist. So, why do these popular explanations live on?

The persistence of prejudices, despite mounting evidence to the contrary, has always fascinated social scientists, who have made some counterintuitive discoveries in their research. This is not an easy question to answer in a few paragraphs. Let me instead refer you to the wonderful new book by Carol Tavris and Elliot Aronson, Mistakes Were Made (But Not by Me): Why We Justify Foolish Beliefs, Bad Decisions, and Hurtful Acts (Orlando: Harcourt, 2007).