Writer Graeme Wood has studied its theology, talked to its students and adherents in the West and describes an apocalyptic insanity that welcomes every Western country into the fight as a fulfilment of the Prophet's words, bringing on the end of time.
Wood admits to a guilty fascination with the internal logic of it all. He writes of one of those he interviewed, an Australian: "He has the kind of unkempt facial hair one sees on certain overgrown fans of The Lord of the Rings, and his obsession with Islamic apocalypticism felt familiar. He seemed to be living out a drama that looks, from an outsider's perspective, like a medieval fantasy novel, only with real blood."
It will be no fantasy for those inside the Islamic State. Muslim "apostates" who do not share its vision pose a greater threat than mere infidels, and can expect a worse fate.
Western governments and media use the word "terror" too lightly. It is nothing that their audiences have experienced. Real terror is living under a regime that needs to keep a population scared to maintain its position. Random arbitrary executions are a political necessity.
Isis is the first reign of terror to publicise its murderous capacity proudly. It does so to provoke an international response, and to respond might boost its recruitment in Islamic countries and communities in the West. But on that score Isis wins either way. If their challenge is not taken up, their potential recruits will think the world is cowed by them.
That is not a good reason to respond, nor is the reason given by governments that have responded, including ours. New Zealanders will be less safe, John Key says, at home and in travels around the world unless we act against this menace.
The only good reason to act is the plight of those under Isis' rule or in danger of its expansion. Western intervention so far seems to be containing its spread and the Atlantic Monthly article suggests containment might be enough.
Wood explains that the "caliphate" needs to expand to fulfil its prophetic mandate. If it can be stopped it will lose legitimacy by its own beliefs and probably self-destruct.
If he is right, New Zealanders will not be risking their lives in vain. But they are not going for our safety, they are joining an action for humanity.
Isis may be capable of inspiring random acts of murder and destruction around the world, as the intelligence agencies tell governments, but security seems to be coping. If those threats were the only consideration it would probably be safer to stay well away from Iraq.
The risk of a New Zealander appearing in a throat-slitting video has just become greater but if it happens, this country's resolve will be hardened.
In the meantime, we are doing what we can to help a population where "offences" can include wearing Western clothes, shaving, voting in an election and failing to name fellow citizens who do not keep these laws.
Wood quotes an article in the Islamic State's magazine that considered whether Yazidis, a Kurdish sect under attack, were lapsed Muslims and therefore had to be killed or merely pagans who ought to be enslaved.
Our soldiers may be on a training assignment but if they can shoot any of these maniacs they should.
