Paris: The Islamic State group has learned from the mistakes of past jihadist movements and established a near-impregnable base of support within Iraq and Syria with spectacular appeal to many of the world's Sunni Muslims, a new book has warned.
The authors of "ISIS: Inside the Army of Terror", published this month in the US, spoke to dozens of fighters and members of the group to understand its allure and how it justifies its brutal tactics.
In a telephone interview with AFP, one of the authors, Syrian-born journalist Hassan Hassan, said it was vital to understand that some of the group's core religious beliefs were widely shared.
"It presents itself as an apocalyptic movement, talking about the end of days, the return of the caliphate and its eventual domination of the world," said Hassan, who lives in Abu Dhabi where he works as a researcher for a think tank.
"These beliefs are not on the margins - they are absolutely mainstream. They are preached by mosques across the world, particularly in the Middle East.
"ISIS takes these existing beliefs and makes them more appealing by offering a project that is happening right now," he said, using an alternative name for IS.
Hassan's research along with co-author Michael Weiss - a US-based journalist - gave them a rare insight into IS training camps for new recruits, which vary in length from two weeks to one year.
"Recruits receive military, political and religious training. They are also trained in counter-intelligence to avoid being infiltrated," said Hassan.
"After they graduate, recruits remain under scrutiny and can be expelled or punished if they show reservations, or sent back to the camps to 'strengthen their faith'."
IS uses certain texts and in-house clerics to provide religious justification for their violence, particularly a book called "The Management of Savagery", which argues that brutality is a useful tool for goading the West into an over-reaction.
The authors outline six categories of IS recruit.
Only two are rooted in religion: the ultra-radicals who dominate the group's upper echelons, and recent converts to its extremist ideology.
Others are merely opportunists seeking money or power; pragmatists who want stability and see IS as the only game in town; and foreign fighters whose motives vary widely but "are almost always fed by serious misapprehensions of what is taking place in Iraq and Syria".