Isis isn’t the first group to use the butcher’s knife as an instrument of policy. Nor will it be the last

Not since the Nazis have we had culprits documenting their war crimes on such a scale

Isis has released a video apparently showing the beheading of a Kurdish man in Iraq as a warning to Kurds fighting the group in Iraq

Chopping off someone’s head or sawing it off or slicing it off is about gore. Blood. Pain. Grotesquerie. Death by the blade is about shame, suffering in an animal slaughterhouse. It’s the most repulsive theatre, understood by the Romans, the Tudors, the French revolutionaries, the austerest guardians of Wahhabism. Colour it bright red.

To be hanged, drawn and quartered was about fear, terror – a word liberally used in Paris after 1789 – and obedience. It still is. The most judicious, accurate and gruesome description I have ever read of such executions – those of a nervous disposition need read no further – came from an expatriate Irishman who chanced upon the “judicial” beheading of three Saudis in Jeddah in 1997.

“Standing to the left of the first prisoner, and a little behind him, the executioner focused upon his quarry. I watched as the sword was drawn back with the right hand. A one-handed back swing of a golf club came to mind … The down-swing begins. How can he do it from that angle? … The blade met the neck and cut through it like…a heavy cleaver cutting through a melon … a crisp, moist smack. The head fell and rolled a little. The torso slumped neatly. I see now Why they tied wrists to feet … the brain had no time to tell the heart to stop, and the final beat pumped a gush of blood out of the headless torso on to the plinth.”

Oddly, back then – in the days when decapitation was regarded as a mundane if unpleasant ritual in Wahhabi Saudi society – this description, in The Irish Times of all places, elicited not the slightest response. No one worried about the sins of the three poor wretches, nor the “trial” they underwent, nor the pain they must have endured. It was all part of a timeless tradition. You know, these warrior chaps, always chopping off bits of one another. Decapitations, amputations, you name it.

Now that the habit has stretched across the deserts to Iraq and Syria, however, and embraced the good, the bad, the ugly and the truly innocent, we’re all talking about genocide, apocalypse and the end of the world. Isis, the latest Middle Eastern plague we have to fear and loathe – remember Khomeini’s hangmen, Saddam’s torturers and Assad’s executioners? – has quite deliberately turned to the butcher’s knife as an instrument of policy. Debate, discussion, objections have no place in the polity of this Salafist lot.

It’s rule by fear, Ghengis Khan-style, Tamerlane the victorious – is it not passing brave to ride in triumph through Mosul? – in which power (and revenge) is imposed through the knife. Shia Iraqi soldiers? Shoot them in the back of the head by the battalion. Christians? Convert or die. Syrian recruits? Strip them and slice their throats. And videotape the whole gruesome business. Not since the Wehrmacht took tourist snapshots of their massacres of the Jews of the Soviet Union have we had culprits documenting their own war crimes on such a scale. Indeed, the mobile phone video, the blog and the internet have become the new purveyors of earthly terror.

There’s no point in searching for the dark inspiration behind decapitation. Almost every ancient text can be used to justify judicial murder, ethnic cleansing or genocide. The Bible is packed with the stuff. But the unique element about Isis – true to the bleak 18th-century philosophy of Muhammad Ibn Abdul Wahhab himself, so harsh and intolerant that the people of Basra threw him out of their city after his brief visit to what is now Iraq – is the idea of a return to the origins of Islam, to purity. Which means pre-schism Islam, before the great Shia divide. And purity is about absolutes, absolute right and absolute wrong, which is why the flag of Isis is black and white – as was the flag of al-Qa’ida.

Of course, the original al-Qa’ida favoured the men who would create this monster. When Abu Musab al-Zarqawi, al-Qa’ida’s man in Iraq, was killed in a US air raid in 2006, Osama bin Laden described him as “a lion of jihad”. But via his successor Abu Abdullah al-Rashid al-Baghdadi and now Abu Bakr al-Baghdadi, this particular al-Qa’ida clone moved out of control. Far from claiming to represent all Muslims, al-Qa’ida’s local affiliates espoused Sunni – even tribal – aspirations. Thus a letter written – probably by Bin Laden himself, less than a year before his assassination by the Americans – complains that some of his “brothers” had become “totally absorbed in fighting our local enemies” and using other Muslims as human shields (Bin Laden called this the “barricade argument”).

Addressing one of his advisers, Bin Laden specifically questioned the actions of “our brother Abu Bakr al-Baghdadi”, demanding that “several sources” be questioned about him – as well as about his Islamic State lieutenant al-Nasir li-Din Allah. “In these efforts,” Bin Laden wrote, “there should be a special message directed to our brothers there that stresses the importance of unity and collectiveness and that they maintain a basic foundation of the religion so it must get precedence over names, titles or entities if they obstruct the achievement of that great duty.” In other words, the al-Qa’ida leader was having grave doubts about Isis, its leadership and its role.

The last critical paragraph of this letter, which passed through the hands of the Americans who found it in Bin Laden’s compound in 2011 after they killed him in Abbottabad – so there’s a red light over the sourcing – begins: “We must avoid the stigma of being a one-dimensional sect [sic], opposed to all others. We are Muslims following the teachings of Islam and we are not the owners of the Salafist way … It is important to have a memorandum … clarifying the issues of penitence … and the virtue of patience; refraining from accusing and judging without being qualified to judge.” Bin Laden even wanted al-Qa’ida to apologise when Muslims were wrongly killed by al-Qa’ida surrogates.

If only we had captured this man and put him on trial for al-Qa’ida’s crimes against humanity – rather than murdered him, which we did – perhaps due process would have allowed us to hear more of Bin Laden’s argument. But of course, we liquidated him. And now America’s military bosses are talking hysterically about apocalypse and their President admits he doesn’t “have a strategy yet”, at least not until he can “cobble together the kind of coalition we need …” And so the blood-dimmed tide is loosed.