It has now become a bitterly regular scenario: news of the disappearance of one or more British citizens, apparently to join Islamic State, or the announcement of the deaths of those who already did so. Distraught families in Bradford raised the alarm this week when three sisters and their nine children – aged between three and 15 – failed to return from a pilgrimage to Saudi Arabia. On 9 June they are thought to have boarded a flight to Istanbul in Turkey – often used as the route into Syria – and nothing has been heard from them since. The brother of the women is already thought to be fighting with Isis.

Shortly before that, it was revealed that Talha Asmal, a 17-year-old from Dewsbury, had become Britain’s youngest suicide bomber after blowing himself up near an Iraqi oil refinery, and that Thomas Evans, a 25-year-old convert to Islam from Buckinghamshire, had died while fighting with al-Shabaab (the terrorist groupresponsible for many atrocities including the Nairobi mall attack).

To the admirers of militant Islamism surrounded by western culture, Isis promotes a satisfyingly fanatical ideology

The observations frequently made following such news, from the media and devastated families alike, are that those involved have been “duped”, “fooled”, “groomed”, and “brainwashed” by radicals. The recruits are frequently described, no doubt accurately, in a domestic context, as pleasant and thoughtful family members and friends. It is natural that the British families of Isis recruits should wish to believe that their relatives have somehow been ideologically deceived into joining. Yet if we go along unquestioningly with that perception, we are also deceiving ourselves. Whatever other charges could be laid at the door of Isis, concealing its true nature is not one of them.

Isis is unusual among terrorist organisations in that it came to prominence extremely quickly, has expanded the territory it controls rapidly, and has been utterly unapologetic about extreme brutality. In the past, terrorist groups have often presented their atrocities as unpalatable but necessary staging posts on the way to an endgame. The Troubles in Northern Ireland, where I grew up, were distinguished not only by sustained paramilitary violence, but by the pervasive stench of hypocrisy from its perpetrators. Whether the IRA was blowing up Protestants queuing in a fish shop, or the UVF was gunning down blameless Catholics in a pub, members took time out for tortuous, if wholly incredible, explanations as to why what they had done was strategic rather than sectarian.

Isis doesn’t bother greatly with hypocrisy, however: that would imply a residual acknowledgement of liberal values in the first place. It openly defends the enslavement, sale and systematic rape of Yazidi women by Isis fighters. It posts pictures of Isis zealots throwing allegedly gay men off high buildings to their deaths. It shoots, decapitates, or immolates prisoners of war, and publicises its mass beheading of captured Coptic Christians because of their faith. It compels women to wear the full veil in public or be flogged. It glorifies violent death as martyrdom, despises other religions and cultures, and is happily intent upon erasing their most ancient history. Name a single liberal value, and Isis is in open opposition to it.

Isis members, including its energetic female propagandists, make no secret of this. One website run by a jihadist calling herself al-Khanssa, for example, is full of pious exhortations and homely advice, but very clear on the basics. A questioner asks: “What do you think about the killing of Steven Sotloff?” (the American-Israeli journalist beheaded by Isis last September). She replies: “I wish I did it.” Another inquires as to whether homosexuals could engage in same-sex relationships under sharia law. The reply comes: “If they want a death sentence they could do whatever they want.”

The appeal of Isis internationally has undoubtedly been boosted by the perception that it is unstoppably establishing itself as a de facto state. One way to curtail that is to arrest or reverse its expansion – as Kurdish fighters have managed to do in Kobane and Tal Abyad. Yet to the admirers of militant Islamism surrounded by western culture – currently tying itself in anxious knots over delicate questions of class, race and gender – Isis promotes a satisfyingly fanatical revolutionary ideology in which (to borrow its own terminology) the lions and lionesses of the caliphate rear bloodthirsty cubs full of roaring certainties. Those drawn to Isis are not deluded as to its intolerant and brutal nature: that is precisely what they find appealing, along with the promise of power, adventure and a glittering afterlife.

It may be disturbing that in so many apparently ordinary British citizens the superficial bank of inculcated liberal, rational values can crumble and scatter as easily as the Iraqi army in Mosul, but we had better recognise that it does. Otherwise, we are choosing only to dupe ourselves.