It is unbearable to think that the unique urban landscape Gertrude Bell, Lawrence of Arabia and Agatha Christie described a century ago no longer exists. But Walpole’s thought experiment, in which a traveller from Lima “gives a description of the ruins of St Paul’s, like the editions of Balbec and Palmyra”, no longer seems such a remote fantasy. The same atavistic theology that now justifies the levelling of the Temple of Bel also encompasses the destruction of churches and synagogues on an even grander scale. We need to recall Gibbon’s similar jeu d’esprit when he considers what might have happened if Charles Martel had not stopped the advance of Islam into Europe at the Battle of Tours: “Perhaps the interpretation of the Koran would now be taught in the schools of Oxford, and her pulpits might demonstrate to a circumcised people the truth and sanctity of the revelation of Mahomet.”

The full significance of the demolition of Palmyra thus only emerges when we consider what it implies about the perpetrators’ attitude to Western civilisation. Ruins that had stood for nearly 1,800 years mean less than nothing to the genocidal ghouls of the new Caliphate, whose aim is to throw history into reverse and annihilate even the memory of all non-Islamic cultures. By harnessing the resources of Western culture — not only military technology but above all using the internet as a propaganda tool — the marauders of Isis have forced themselves into the forefront of our consciousness. Islamism is the face of nihilism in our time. The paralysis of the Western democracies when confronted with such radical evil is not unprecedented — we did not stop the Holocaust or the Cultural Revolution either — but what is new seems to be the brazen self-aggrandisement of the perpetrators. The great crimes of the 20th century were largely hidden from the world while they took place. This time, Isis has forced us to watch the agony of a civilisation. Whose civilisation is it? Ours — for the ruins of Palmyra belong to our cultural heritage no less than their architectural progeny, the English country house or the Capitol. The casual murder of Khaled al-Asaad in front of the antiquities that had been his life’s work recalls the death of Archimedes, who according to Plutarch was slain in Syracuse by a Roman soldier because he would not look up from his geometrical diagrams in the dust. Yet the Roman general, Marcus Claudius Marcellus, was apparently furious, having given orders that Archimedes was not to be harmed.

The Romans often behaved in a barbaric way — for example, by reducing Palmyra to a ruin — but they were not barbarians. The Islamists of the new Caliphate glory in their barbarism. They also have a growing number of admirers and apologists here. Once Isis has finished with Palmyra, the media caravan will move on to another oasis of death, with a new horror show to fill our screens. But the voyeuristic atrocities we are witnessing in Syria and Iraq are a foretaste of what the future has in store for the West — including Britain — unless we act now.