It has been ugly. Two weeks of full-on, culture-wars inspired, anti-intellectualism; a fortnight of alternative-facts, dog-whistle racism and shameless misogyny. Yet this – one of the nastiest Twitter rows to date – was sparked by, of all things, the emergence of a children’s cartoon set in ancient Rome.

Hostilities commenced when Paul Joseph Watson, who goes by the name @PrisonPlanet on Twitter, attacked a BBC cartoon. His issue was that the father of the central family was portrayed as dark skinned. Sensing a politically correct plot to take over British history, one presumably orchestrated by the liberal elite from somewhere deep within their headquarters in the out-of-touch, metropolitan, media bubble, Watson went on the offensive. “I mean, who cares about historical accuracy, right?” he tweeted. Battle lines were drawn when former teacher Mike Stuchbery responded by pointing out that “Roman Britain was ethnically diverse, almost by design”. From there it rumbled on.

Yet even as the online phalanxes crashed into one another, I presumed, naively as it turns out, that when one of the world’s foremost experts on the Roman empire, a professor of classics at the University of Cambridge no less, offered her judgment on the matter all but the most rabid culture warriors would accept that this was a false alarm and stand down the troops. Not a bit of it, because not only had Professor Mary Beard taken away their political football, she’d done so while being openly and inexcusably female.

In space, so they say, no one can hear you scream. Online, many people seem unable to hear facts, even when they carefully laid out by a renowned expert. I am not a classicist. Which is why I defer to the scholarship of academics such as Beard when it comes to Roman history, but everything I have read leads me to conclude that there is a broad consensus of academic opinion that there were people who lived in Roman Britain who would fit the modern definition of “black”. Not that the Romans recognised race in modern terms nor recorded it in the records they left us.

At its height, Rome’s empire stretched right along the coast of north Africa and sub-Saharan Africans passed to and fro across its porous southern border. The archaeological evidence, much of it based on relatively new forensic techniques such as isotope analysis, reinforces the historical record, indicating that Africans from both above and below the Sahara made their homes and built their lives in the British Isles. It has been research such as this that has given us the “ivory bangle lady”, a well-to-do, part-African resident of 3d-century York. More recently, the “Beachy Head lady”, the first black Briton known to us, has been discovered using a similar suite of forensic techniques. None of these remarkable discoveries or any of the other evidence had much purchase on Twitter.

Facebook Twitter Pinterest Mary Beard, who has been vilified online. Photograph: Caterina Turroni/BBC

Were this just another case of the online angry brigade attacking Beard for being knowledgeable and female at the same time, it would have been unpleasant but not new. What was novel here was that the American economist and philosopher Nassim Nicholas Taleb, presumably at something of a loose end, took time out to lead the charge against Beard, accusing her of “talking bullshit”. In a moment of near hysteria, Taleb announced that an online row about the accuracy of a fictional character in a children’s cartoon was definitive proof that “scholarship is dead in the UK”.

To be optimistic for a moment, we need to remember that thousands of people rushed to Beard’s defence and continue to do so. A number of famous voices also leant their support and lamented that, again, a women in the public eye has become the focus for a storm of vile abuse. It is also comforting to remember that beyond Twitter, most people, in my experience at least, regard the fact that Britain under the Roman empire was a more racially diverse society than we once thought as little more than a fascinating historical detail. It’s one of those surprising facts that gets less surprising once you start thinking about it. We know the Roman empire contained people from three continents and we know that the Romans loved to travel, as demonstrated by the thousands of miles of arrow-straight roads they left behind them, all of them famously leading to Rome itself. But the events of the past three weeks should be seen as part of pattern. Similar, although far less aggressive denouncements have been made in the past against those who have sought to portray the presence of black people in eras of British history before the Second World War.

In 2007, Doctor Who, then in the form of David Tennant, took a trip to Shakespeare’s London in an episode set in 1599. The depiction of the Elizabethan capital, replete with its small black population, led to another charge of historical inaccuracy. The programme makers were accused of distorting British history in the name of political correctness. Sound familiar?

The online campaign against historical diversity raised its banners again earlier this year. For a second time, their target was the time lord, who by then had regenerated into the more grizzled figure of Peter Capaldi. This time, the doctor had time-travelled to regency London and again black faces could be seen in the crowd. Walking around the London of 1814 the doctor’s companion, Bill Potts, played by the mixed-race Pearl Mackie, noted that the city was “a bit more black than they show in the movies”. “So was Jesus,” quipped the doctor. “History’s a whitewash.” On both occasions, the historical evidence upon which the writers based these scenes is uncontested. Yet still accusations of historical inaccuracy were levelled and angry voices raised online.

What we’re seeing is a backlash against any attempt, whether from the world of scholarship or popular culture, to paint non-white people back into the British past. Those of us who write about this history have long been familiar with this. In the 1990s, an assistant in a London bookshop informed the African American historian Gretchen Gerzina that there “were no black people in England before 1945”. Gerzina rather effectively disproved that assertion by going on to write the classic book on black people in Georgian London, Black London.

The deeper, more fundamental question is why? Why are some people so affronted by the very idea that the black presence in Britain stretches back so many centuries? Why, even when historical evidence is presented and the opinions of experts given, are they determined to dismiss the facts and, as we have seen in this case, seek to trash the reputation of respected scholars? The refusal to accept that the black presence in Britain has a long and deep history is not just a symptom of racism, it is a form of racism. It is part of a rearguard and increasingly unsustainable defence of a fantasy monochrome version of British history.