You’ll remember Godwin’s law, which holds that the longer an online debate goes on, the likelier it is that someone will mention Hitler or the Nazis. It was an amusing observation and one that served a useful purpose, guarding against hyperbole and fatuous comparison. Except last August, as the American far right staged a torchlight parade in Charlottesville, Mike Godwin suspended his own law. “By all means, compare these shitheads to Nazis,” he tweeted. “Again and again. I’m with you.”

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Despite that dispensation, I’ve tended to abide by my own version of Godwin’s law. I try to avoid Nazi comparisons, chiefly because they’re almost always wrong and because, far from dramatising whatever horror is under way, they usually serve to minimise the one that killed millions in the 1940s. And yet, there’s a cost to such self-restraint. Because if the Nazi era is placed off limits, seen as so far outside the realm of regular human experience that it might as well have happened on a distant planet – Planet Auschwitz – then we risk failure to learn its lessons. That would be to squander the essential benefit offered by study of the Third Reich: an early warning system.

So yes, when Donald Trump ordered US government agents on the southern border to separate migrant children from their parents, to tear screaming toddlers from their fathers and even to pull a baby from its mother’s breast, he was not re-enacting the Holocaust. He was not ordering the eradication of an entire people or sending millions to their deaths. But there were echoes. And we must hear them.

For one, there’s the elemental act of separation itself. If you interview survivors of the Holocaust, one thing you notice is that even those who’ve grown used to describing events of the most extraordinary cruelty, and who can do so without shedding a tear, often struggle when they recall the moment they were parted from a parent. Mostly now in their 80s or older, they are taken back to that moment of childhood terror, one that never leaves them.

The parents ripped from those 2,300 children on the Mexican border were not led off to be murdered. But there are grounds to believe they may never again see their sons or daughters, some of whom were sent thousands of miles away. There is no system in place to reunite them. The children were not properly registered. How can a two-year-old who speaks no English explain who she is? Eighty years from now, perhaps, old men and women will sob as they recall the mother taken from them by uniformed agents of the US government, never to be seen again.

But the echoes don’t end there. The wire cages. The guards telling weeping children they are forbidden from hugging each other. And then this chilling detail, reported by Texas Monthly. It turns out that US border guards don’t always tell parents they’re taking their children away. “Instead, the officers say, ‘I’m going to take your child to get bathed.’ The child goes off, and in a half-hour, 20 minutes, the parent inquires, ‘Where is my five-year-old?’ ‘Where’s my seven-year-old?’ ‘This is a long bath.’ And [the officer says], ‘You won’t be seeing your child again.’” It’s not the same as telling Jews about to die they are merely taking a shower, but in the use of deception the echo is loud.

And if the mechanics of this operation strike a familiar note, so too does the rhetoric and propaganda deployed by those behind it and defending it. You don’t have to go to back to 1930s Germany to know that the first step towards catastrophe is the dehumanisation of a reviled group. It happened that way in Rwanda and the Balkans in the 1990s, and it’s happening in today’s United States. “These aren’t people, these are animals,” the US president said last month. They want “to pour into and infest our country”, he tweeted this week. “Infest” is a word reserved for rats and insects. This is the language of those seeking to choke off human sympathy, by suggesting those suffering are not even human.

Trump’s defenders reinforce the message. It was a jolt to see Steve Hilton, onetime shoeless guru of David Cameron’s Downing Street, now reinvented as a Fox News host, grinning away as pundit Ann Coulter called the crying infants “child actors”. Her message was repeated on Fox by Nigel Farage, who similarly urged Trump not to be swayed by the “screams coming from the liberal media” and to “stay tough”.

Farage is a reminder that this phenomenon is not confined to the US. Referring to refugees, Italy’s new interior minister, Matteo Salvini, has called for a purification, or perhaps a cleansing, of his country, “neighbourhood by neighbourhood, street by street”. His plan is to draw up a register of Roma living in Italy. Those with Italian citizenship, “we’ll have to keep, unfortunately”, he said.

The signs are there, if only we can bear to look. Something is happening to our world. Others have noted the way the post-1945 global architecture is beginning to crumble, as Trump undermines the western alliance in favour of authoritarian tyrannies. But the postwar order is unravelling in another, more insidious way too.

Put starkly, the norms and taboos established after the world witnessed the Holocaust are eroding before our eyes. For 70-odd years, roughly the span of a human life, they endured, keeping the lid on the darker impulses that, we had seen, lurked within all of us. It steadily became taboo to voice undiluted racism and xenophobia. Those fears, those loathings of the stranger, never went away, of course. But they were held in check, partly by the knowledge of where such hatred, unrestrained, could lead.

Now, in the US, Italy, Hungary, Poland and elsewhere, the restraints are off. There even seems to be a macho thrill in breaking the taboo, in echoing the words and deeds of that darkest era in human history. It’s as if the boundaries that were drawn after 1945, demarcating acceptable human behaviour, were mere lines in the sand – and now the tide is coming in.

It doesn’t happen overnight. It happens bit by bit, word by word, each step taking us lower into the pit. It’s why every one of us has to fight today’s horror. Because if we don’t, who knows what terrors lie ahead?

• Jonathan Freedland is a Guardian columnist