In recent years, high-level Western officials have argued that terrorism is a product of poverty, a lack of education or mental illness. Other influential voices have urged that terrorist acts expose the truth about Islam, and still others that they are a natural, if excessive, response to legitimate grievances against the West.
On Monday, British Prime Minister David Cameron pointedly rejected every one of these theories - and went on to provide what may well be the most clear-headed explanation ever offered by a head of state.
Poverty and disadvantage can't explain violent extremism, Mr Cameron explained, because many terrorists "have had the full advantages of prosperous families or a Western university education".
In his view, the root cause of terrorism is instead an extremist ideology, fuelled by a process of radicalisation. "No one becomes a terrorist from a standing start", is how he put it. The problem lies in how information spreads, above all within enclaves of like-minded people.
Conspiracy theories provide a gateway. Young people are repeatedly told that Jews wield malevolent power, that Israel was behind the Sept 11 attacks, and that Western powers are trying to destroy Islam.
Theories of this kind breed a sense of humiliation and rage; they drown out "many strong, positive Muslim voices", Mr Cameron said. Social isolation is a big part of the picture. Despite the United Kingdom's success in creating a multi-faith, multi-ethnic democracy, many of its citizens do not identify with their country or with fellow citizens who belong to other groups.
Terrorist groups are cults, by Mr Cameron's reckoning, whose members listen and talk mostly to one another. Social scientists show that isolated enclaves produce "group polarisation" - a process by which like-minded people, interacting mostly or only with one another, go to extremes. Within extremist groups, this process leads to violence.
The Prime Minister's remedies follow from his diagnosis. He wants to respond to baseless conspiracy theories and take the glamour out of the extremist cause - a cause that embraces such acts as men raping underage girls. To that end, he plans to empower Britain's Syrian, Iraqi and Kurdish communities to speak out against the violence. He will encourage the Office of Communications to take steps "against foreign channels that broadcast hate preachers and extremist content". Mr Cameron insists that "this extremist ideology is not true Islam", noting that this needs to be said clearly, in opposition both to the extremists and to ugly, counterproductive Western voices that attack the religion as such.
At the same time, extremists do proclaim their fidelity to the Muslim faith, so action must be taken to empower the moderate voices. "We are with you, and we will back you," Mr Cameron says to them, "with practical help, with funding, with campaigns, with protection and with political representation."
He wants the UK to take a broad look at how to reduce social segregation in both education and housing.
He also wants help from the private sector in the effort to deradicalise potential extremists. He asks broadcasters, for example, to consider featuring Muslims who abhor violence - and who "have a proper claim to represent liberal values in local communities".
And Mr Cameron wants Internet providers to do more to help identify potential terrorists online.
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