The guide shook with anger. A tourist, ignoring the notices, had taken a flash photograph of an ancient wall painting in a 3rd-century tomb in Palmyra, Syria. For this, he was given a brief, fierce lecture on the importance and fragility of the work and on respect for it.

I wonder about and fear for the guide, as I do about other people I met in Syria, especially in those areas now controlled by Islamic State – a pair of hopeful teachers in Deir ez-Zor; a cafe owner near Raqqa who displayed large pictures of the Assad family. This was probably not due to any love for the president, but because he would have been given little choice in the matter. The guide will have lost his livelihood, but it is hard to imagine his horror now, as Isis threaten to capture his home town of Palmyra.

A camera flash, understandably upsetting to the guide, would be as nothing compared with the destruction that Isis would do there. As they have to Nimrud in Iraq, and to many of Mosul’s monuments, they will try to obliterate it: smash its carvings, deface its paintings, topple its columns with bulldozers, atomise its masonry with explosives.

Palmyra is an ancient Roman site whose significance and value is exceeded by very few others: those in Rome itself, Pompeii, possibly Petra in Jordan. Its temples, colonnades and tombs, its theatre and streets are extensive, exquisite, distinctive, rich. The loss of Palmyra would be a cultural atrocity greater than the destruction of the Buddhas in Bamiyan. It is hard to think of deliberate vandalism to equal it, despite the grim examples offered by the last hundred years.

I first went to Palmyra and Syria as a student, in 1982, on an impulsive detour from a trip round Turkey. I was blissfully unaware of the fighting earlier that year between the current president’s father and the Muslim Brotherhood that left many thousands dead, but which now looks like a minor squabble compared with the current civil war. Other tourists must have been better informed, as my travelling companion and I had the ruins to ourselves.

I toured the country again in 2009. By then, the place was more popular, with fat tourist buses winding up to its overlooking Mamluk castle, which, reports suggest, Isis forces have now reached, to catch the sunset view.

The country was astonishing, not just for the splendour and beauty of its treasures, but for the range and richness of cultures they represented – Roman and Byzantine cities, Arab souks and palaces, Ottoman hammam, Norman castles, synagogues, churches, mosques and shrines, Sunni and Shia. Also, Hittite and Assyrian remains, going back five millennia. One of the ghastly ironies of the current violence is that the places where civilisation started are now the world’s most barbarous.

Although they were often built under the influence of foreign powers, these places were also Syrian and helped shape Syria’s identity before the war. Palmyra exemplifies this: although described as Roman, most of what is there now was the creation of an essentially self-governing state that grew rich and powerful through its position on trade routes. Its temples served local gods and its Roman architectural style is combined with regional variations. Its inscriptions are in a dialect of Aramaic as well as Latin and Greek. It was made by Syrian people whose faces are, for now, vividly recorded in its paintings and carvings.

I remember vividly the wonderful city of Aleppo, sometimes described as the oldest continually inhabited city in the world, with its courtyarded houses and a sequence of contrasts that ran from its citadel through its souk to its mosque, which is 8th century in origin.

The citadel was like a fortification from legend, a huge walled rock reached up a steep bridge and through massive gates. The souk was a fabric of human activity, its spaces made by actions and produce (spices, soaps, meat, medicines, hardware) more than by stone, but in which churning of the here and now was relieved by shrines and other moments of calm, and by ornaments or domes that would connect the present with the past. The mosque’s courtyard was the opposite of the souk – an orderly, ample rectangle of peace, paved in clean and shining stone, in which people would sit at perfect ease, as if it were their living room.

As the war has progressed, there has been a miserable drip of dreadful news about these places, caused not only by Isis but also by the violence of the government forces and other players. The Aleppo souk has been devastated, the mosque’s minaret destroyed. The Seleucid city of Dura Europos, founded in 303 BC on a site overlooking the Euphrates, has been violently dug up in the search for antiquities that can be sold to help finance Isis. Palmyra itself had already been plundered and damaged before the latest threat emerged.

There have been more modest losses: Deir ez-Zor, which is not otherwise greatly blessed, had a nice footbridge over its river, which made for delightful evening strolls for its citizens. There will be other places of which I don’t have news – a desert palace seen in a storm of red dust; a lonely and vulnerable monastery in a cleft in a cliff; a ruined city.

If Isis raze Palmyra, it would be a new demonstration of the evil and stupidity they have already abundantly displayed in their slaughters and enslavements, and in their videos of beheadings and burnings. It would also confirm Isis’s littleness: how could anyone be so threatened by ancient ruins, unless they lacked belief in their ability to create something themselves? It would make manifest Isis’s nihilism, their vision of the world as a desert populated only by themselves and their slaves. It is, of course, precisely the diversity of Syria’s heritage that Isis hate.

It would kill a piece of the soul of Syria and of hope for its future when the war finally ends. It would also destroy something that belongs to the whole world. For this reason, surely, there is a case for the American-led coalition to defend it with air strikes, as they have defended threatened civilians in Syrian and Iraqi Kurdistan. It would mean a temporary alliance with the forces of the country’s obnoxious government, who are defending Palmyra, but the protection of something this exceptional and precious overrides almost any other consideration.