Here's an excellent article on how people become terrorists by James Q. Wilson. I particularly liked these two particular bits:

Ideological terrorists offer up no clear view of the world they are trying to create. They speak vaguely about bringing people into some new relationship with one another but never tell us what that relationship might be. Their goal is destruction, not creation. To the extent they are Marxists, this vagueness is hardly surprising, since Marx himself never described the world he hoped to create, except with a few glittering but empty generalities.

A further distinction: in Germany, left-wing terrorists, such as the Red Army Faction, were much better educated, had a larger fraction of women as members, and were better organized than were right-wing terrorists. Similar differences have existed in the United States between, say, the Weather Underground and the Aryan Nation. Left-wing terrorists often have a well-rehearsed ideology; right-wing ones are more likely to be pathological.

I am not entirely certain why this difference should exist. One possibility is that right-wing terrorist organizations are looking backward at a world they think has been lost, whereas left-wing ones are looking ahead at a world they hope will arrive. Higher education is useful to those who wish to imagine a future but of little value to those who think they know the past. Leftists get from books and professors a glimpse of the future, and they struggle to create it. Right-wingers base their discontent on a sense of the past, and they work to restore it. To join the Ku Klux Klan or the Aryan Nation, it is only necessary that members suppose that it is good to oppress blacks or Catholics or Jews; to join the Weather Underground, somebody had to teach recruits that bourgeois society is decadent and oppressive.

By contrast, nationalistic and religious terrorists are a very different matter. The fragmentary research that has been done on them makes clear that they are rarely in conflict with their parents; on the contrary, they seek to carry out in extreme ways ideas learned at home. Moreover, they usually have a very good idea of the kind of world they wish to create: it is the world given to them by their religious or nationalistic leaders. These leaders, of course, may completely misrepresent the doctrines they espouse, but the misrepresentation acquires a commanding power.

Yep. If we extend the analogy to Spain, the strongest terrorist group is ETA, which started out as a nationalist terrorist gang with ideological overtones. As ETA matured, it suffered several schisms; in each one, the more ideologically leftist, more intellectual, less violent branch left the organization and gave up violence. The more nationalist branch, who consider themselves to be carrying on the tradition of Sabino de Arana, has continued killing. The GRAPO, a strictly ideological ultraleftist gang, had its glory days at the same time as the Baader-Meinhoffs and their ilk. It never had anything like the mass social support ETA enjoyed (and still does in some places) in the Basque Country. It's still going, but it's down to a few active members. Occasionally they do something really bad like rob an armored car and kill the guards, but they're mostly pretty quiet these days.

In the 1970s, I attended meetings at a learned academy where people wondered what could be done to stop the terrorism of the German Red Army Faction and the Italian Red Brigades. The general conclusion was that no counterattacks would work. To cope with terrorism, my colleagues felt, one must deal with its root causes.

I was not convinced. My doubts stemmed, I suppose, from my own sense that dealing with the alleged root causes of crime would not work as well as simply arresting criminals. After all, we do not know much about the root causes, and most of the root causes we can identify cannot be changed in a free society—or possibly in any society.

The German and Italian authorities, faced with a grave political problem, decided not to change root causes but to arrest the terrorists. That, accompanied by the collapse of East Germany and its support for terrorists, worked. Within a few years the Red Army Faction and the Red Brigades were extinct. In the United States, the Weather Underground died after its leaders were arrested.

But Islamic terrorism poses a much more difficult challenge. These terrorists live and work among people sympathetic to their cause. Those arrested will be replaced; those killed will be honored. Opinion polls in many Islamic nations show great support for anti-Israeli and anti-American terrorists. Terrorists live in a hospitable river. We may have to cope with the river.

Yep. He's right. Nobody but a bunch of squatters, a few radical sociologists, and eight guys with long, dirty beards who smell of cheap wine ever supported the GRAPO. ETA still has solid support across the board in much of the Basque Country. Now, arresting the terrorists has worked very well against both gangs, but ETA has been able to survive because it's the crazy uncle or the black sheep of the Basque nationalist family. My guess is that most Basque nationalists wish ETA would stop with the killing already, but they can't bring themselves to openly condemn their boys, bad boys though they may be.

Read the whole article. It's very interesting.