I thought I knew all about the camps, but I did not know about the field of teeth. I knew the Third Reich from books and films. And you cannot grow up in a Jewish household without knowing about the Holocaust. We have met the reproachful stares of our ancestors, recorded in fading, olive-hued prints from the days when sitting for a photograph was a formal matter. The missing limbs from our family trees ache across the decades.

So I knew Auschwitz-Birkenau before visiting the site last week. I knew about the 1.1 million people who died there. I knew about the selection of those deemed capable of work from the rest, who were sent for extermination. I knew about tangled mounds of round-rimmed spectacles and rolls of fabric woven from human hair. Nothing was casually discarded by the camp administration, apart from humanity. As the sun went down and the freezing wind picked up, our guide led us to the remains of one of the gas chambers. Here was a place where testimony stops because none who entered survived. Here there is no knowledge without a leap of imagination, and imagination flinches.

Then I learned about the teeth. The “special units” of camp inmates who were tasked with disposal of the bodies tried to preserve evidence of the crimes by writing diaries and burying them. They sowed them into the Polish soil, along with teeth that had withstood the incineration of corpses, anticipating one day a harvest of remembrance. And there we stood: a few journalists, a pair of MPs, teachers and students, aged 16-18: some 200 in all, vowing never to forget.

We had been brought to that place by the Holocaust Educational Trust, a charity that provides schools with material resources, study courses and survivor testimony. It is a programme of intellectual vaccination. Exposure to a neutralised strain of some viruses stimulates the body’s defences, generating antibodies in the bloodstream that guard against future infection.

For generations, collective memory of the 20th century’s horrors has performed that function. Britain has unique confidence in its immunity. It flows through the celebration of our resistance to fascism when so much of Europe succumbed. The Churchillian narrative of the solitary fightback between the fall of Paris and the arrival of American reinforcements – Our Finest Hour – has eclipsed the preceding years, when appeasement was government policy. Before Hitler was an emblem of evil, he was a bulwark against the Soviets and the architect of a German economic renaissance, to be handled with diplomatic indulgence.

There is a perverse kind of comfort in dressing present dangers in extreme warnings from the past

Being generous to the appeasers, we might consider that memories of the first world war were fresh. It was difficult to imagine that European civilisation was heading for mechanised slaughter on an even greater scale.It is hard enough to behold the face of that Gorgon today, when the evidence is scattered across the continent. The memory of past horrors is necessary for the prevention of future atrocity, but it is not a reliable guide to the manner of their evolution.

History does not repeat itself any more precisely than you and I are exact repetitions of our parents. Marine Le Pen, leader of France’s National Front, is not the repetition of Jean-Marie Le Pen, who led the same party, who has defended the collaborationist regime of Field Marshal Pétain and belittled the gas chambers as “just a detail” of the second world war. Le Pen the daughter distances herself from Le Pen the father. She reconfigures ultranationalism as the antidote to a decline of French democracy, attributed to its infiltration by Muslim migrants whose religious culture is said to be immiscible with the secular values of the Republic. Her reward is a likely place in the final round of next year’s presidential election.

None of the far-right parties now flourishing across Europe is a straight repetition of the fascist movements that took over when fragile interwar democracies failed to take root, although most mobilise ideas of national exceptionalism that were first mythologised in that heyday of continental extremism.

Modern Russia is not repeating Soviet authoritarianism, although Vladimir Putin has overseen a cultural rehabilitation of Stalin, and applies a policy of institutional amnesia to his crimes. Donald Trump displays the character traits of a monstrous tyrant but he could never be America’s Mussolini, because the US in 2016 is not Italy in 1922. When the Daily Mail runs headlines naming judges as “enemies of the people”, it is not reprinting the same newspaper’s old enthusiasm for the Blackshirts. When Nigel Farage campaigned for Brexit with posters depicting refugees as a menacing horde at the gates he was not reissuing German propaganda from the 1930s, because this was Britain in the 21st century.

To use those darkly familiar idioms is repugnant, inexcusable. But it is more often a rhyming likeness than verbatim recitation. The difference matters. It is tempting to reach for exact parallels because that is where we feel immunity lies. Outraged liberals want to be sure that historical antibodies still course through the nation’s veins, and so we decry every cut to the fabric of democracy as if it were made by the antique knife. There is even a perverse kind of comfort in dressing present dangers in extreme warnings from the past. To conjure the worst that once happened, against which we have resources of comprehension, can obstruct diagnosis of what is happening today. It is vital to remember the bottom of the abyss, but it is harder to navigate the edge if we cry out as if already falling.

There is no precedent for a country with traditions of democracy, free speech and the rule of law as established as they are in Britain or America collapsing into totalitarian darkness. There is huge resilience in our society and our culture. Is there a new, dangerous strain of nationalism in our midst? I think so. Can it overcome historical inoculation? I hope not. Will it adapt to modern conditions, taking a form that is itself essentially modern, unprecedented? Almost certainly.

Fascism was not a regression into a state of ancient savagery. It co-opted science and new industrial techniques for the more efficient pursuit of its aims. It did not reject progress; it claimed to own progress. This is a harder lesson from history: the next threat is the one we struggle to know because it is still beyond the reach of imagination.