He thought he was working in the past tense, making a film about what one Republican-appointed judge described as “one of the most shameful chapters in the history of our country”. Then Donald Trump won the 2024 presidential election. Now Errol Morris’s documentary about family separations at the US-Mexico border looks like a dreadful premonition.

“It’s interesting how things have radically changed,” Morris says via Zoom from a book-lined office in Cambridge, Massachusetts. “The movie, which presumably is recounting past history, seems to be a crystal ball into what may happen next and that was not clearly imagined at the outset. But it is clearly suggested now.”

Separated is based on the NBC News correspondent Jacob Soboroff’s book Separated: Inside an American Tragedy (“one of the best collaborations I’ve ever had”, says the Oscar-winning Morris) and premieres on the MSNBC network on 7 December. It is an excruciatingly timely reminder of how Trump ripped 5,500 children from their parents (up to 1,400 of whom are not yet confirmed as reunited).

The 93-minute documentary forensically details how the first Trump administration’s policy of family separations was deliberate, systematic and intentionally inhumane, leaving children in wire-mesh cages with feelings of fear and abandonment. Trump said with casual cruelty: “When you have that policy, people don’t come. I know it sounds harsh but we have to save our country.”

Wearing white shirt and spectacles, sipping from a white coffee mug and speaking slowly in honeyed tones, Morris reflects: “The separations was an abomination. It was racist, was cruel, was unnecessary. As one of the interviewees in my film says, there were other levers that we could pull. This seemed to be something we did not need to do.”

Trump had come into office promising a crackdown on illegal immigration including the construction of a border wall. The pre-existing catch-and-release scheme (which had allowed migrants to remain in the country until their immigration hearing) was ditched in favour of something more draconian.

Family separations under his administration began as early as March 2017 under a pilot programme in El Paso, Texas. The fact it is was happening covertly undermines the notion that it could act as a deterrent.

A “zero tolerance” policy, officially announced in spring 2018, marked a significant escalation. It mandated the prosecution of all adults crossing the border illegally. Anyone who did not arrive at a designated port of entry and claimed asylum would be arrested.

While the policy never specifically called for children to be taken from parents, separation became inevitable because the adult was detained and charged. Since children were not allowed to be held in a federal jail, they were taken from their parents and placed in the care of the Office of Refugee Resettlement (ORR).

Jonathan White, a civil servant who worked at the ORR and fought against the policy, says in the film: “Harm to children was part of the point. They believed it would terrify families into not coming.”

Images of children held in cages in a McAllen, Texas, facility triggered outrage in June 2018. But Homeland security secretary Kirstjen Nielsen denied that there was a policy of separating families at the border and sought to shift blame to Congress, even though the enforcement of laws happens at the president’s discretion. The Bush and Obama administrations had largely allowed families to stay together.

Morris comments: “There was a totally fatuous claim that is made by Kirstjen Nielsen in the film: we’re just following the law – if you arrest a criminal and they have a child with him or her, you separate them.

“There have been miserable policies towards immigrants from probably every administration, the first Bush on through to Donald Trump. But none of those administrations felt the need to do what he did. It was considered to be a step too far, a no-no, and yet they embraced it anyway.”

He continues: “There are a lot of things that get to me but what really appalls me is that they would separate nursing infants from their mothers. This is clearly not right. What’s the word I’m searching for? This is wrong.”

For Morris, the child separation saga pointed to a wider issue. “It’s an issue about racism and what I see as the racist rhetoric and policies of Donald Trump and his acolytes. I find it repulsive. I often like to remind people that racism is disgusting and it’s also bad manners. Haven’t we been taught not to act like that? Isn’t that part of the repertoire of being a civilised, cultured human being?

“I hate analogies, but like everyone else, I can’t avoid using them. I like to tell people, as an American Jew, I always wondered what it was like to live in Germany in the 1930s, more specifically to be a Jew living in Germany in the 1930s. Now I know a lot more about what it must have been like.”

Morris’s works include The Thin Blue Line, The Fog of War, American Dharma and My Psychedelic Love Story. Separated came with some distinct challenges. Much of the separation process happened away from TV cameras; the director compensations with dramatisations to portray a Guatemalan mother and son experiencing the border crossing, separation and reunification.

It was also hard to get interviews with those involved. Morris explains: “There are all kinds of impediments to getting people to talk; I’ve never seen anything this severe. If you’re working for the government, for example, like Jonathan White was working for the government, you’re constrained. You’re not allowed to talk without getting the permission of your superiors.

“Most people who are still working in some capacity for the government simply would not talk and it didn’t matter how much begging and how much cajoling I might do. Jonathan would and that represents an extraordinary act of courage on his part.

“He felt that the issues were so important that he had to talk. Call him a whistleblower. Call him whatever you want to call him. He did something that was incorrect and greatly appreciated by me. He took risks in order to tell a story which I believe needed to be told. A hero.”

In his interview White describes Scott Lloyd, the head of the ORR, as “the most prolific child abuser in modern American history”, given Lloyd’s disturbing lack of awareness of the trauma inflicted on children under his care.

Morris reflects: “Why is he doing the job? He’s a political appointee. He was known for his anti-abortion activism and that was his chief concern: preventing any of the women in ORR custody from ever getting abortions, even though at that time Roe was the law of the land.

“Was Scott Lloyd interested very much in the care of people in his charge? I don’t know. It seems to me – I hate to make these inferences but I don’t hate them so much that I’m unwilling to make them – that he was currying favour with the administration. He was interested in self-advancement. He was ambitious and perfectly willing to do the bidding of the hardliners in the Trump administration.”

Separated is also a study in the bureaucratic machinations behind how the sausage is made. “There is a very strong theme running through this about bureaucrats and bureaucracy, good bureaucrats and bad bureaucrats. Most interesting to me in the story is how pliable our morality is.

“If we need to find a way to justify the most appalling behaviour, we somehow find a way to do it. You can listen to Kirstjen Nielsen braying like a donkey that she is just following the law – you wouldn’t want me to break the law, would you?

“Well, I don’t look at it that way and, when it’s suggested that she might be separating families as a deterrent to immigration, she gets outraged. I can’t even believe you would suggest such a thing. This is all Looney Tunes. It’s people living in some strange nimbus of self-deception.”

The film highlights the role of civil servants who challenged the policy and fought to reunite families – courageous individuals such as White and Jallyn Sualog who worked within the system to mitigate its harmful effects. And it offers a reminder of the mass street protests – plus worldwide condemnation from the pope and others – that ultimately compelled Trump to back down: the one significant policy reversal of his first term.

Yet a scandal that has been called “torture”, and by Morris himself as leading to “state-created orphans”, gained relatively little attention during this year’s presidential election campaign. Democrats were on the defensive on the border issue and tried to avoid the subject.

Morris says: “People were scared to talk about immigration. The Democrats were and the Republicans weren’t scared to talk about it as long as they could frame it in the most draconian, repulsive terms: we’ll deport everybody.”

He was denied a chance to help put the issue on the agenda when Separated was not scheduled for TV broadcast until after election day. Morris complained on the X social media platform: “Why is my movie not being shown on NBC prior to the election? It is not a partisan movie. It’s about a policy that was disgusting and should not be allowed to happen again. Make your own inferences.”

Trump claimed that undocumented migrants are “poisoning the blood of our country” and asserted, without evidence, that Haitians were eating pet cats and dogs in Springfield, Ohio. He pledged the biggest mass deportation in US history and has already announced a team including the immigration hardliners Tom Homan and Stephen Miller, both of whom were instrumental in family separations during the first term.

Will there be another public revolt this time or, given Trump’s victory in the national popular vote, are people demoralised and desensitised? Morris asks: “Did people in Germany all know that there was antisemitism? Well, yes. Did they know that they were involved in genocide? Probably not everybody.

“On the part of the public, there’s a concept I’m very fond of: anti-curiosity. I sometimes say to myself, how much will it cost me to know less? There’s denial, there’s self-deception, there’s willful disbelief and on and on and on and on and on. I often say Homo sapiens: very bad and most certainly a compromised species.”

But a mass deportation operation will be costly, logistically difficult and likely to produce harrowing images on TV that could reignite the anti-Trump resistance. At a recent screening of Separated in Washington, an audience member interrupted Soboroff and others on a panel discussion by shouting: “We’re not going to let him make our federal government the Third Reich of the US! We’re not going to let him make our National Guard people the Gestapo of the United States! We are not going to let that happen!”

The sequel is always worse. Mass deportations would mean a return to child separations by another name. Some 4.4 million US citizen children lived with at least one undocumented parent as of 2018. The return of Trump has left Morris thinking about questions of justice.

“What happens when you have crime without punishment?” he asks. “We all have this kind of quasi-religious model that moral transgressions have to be punished. There has to be some kind of societal reply. But what if there isn’t? What if crime goes unpunished?

“I was just in Ukraine and I kept wondering – they’ve recorded over 100,000 war crimes by Russian soldiers – will these go unpunished? Will there ever be any kind of accountability? My answer to that is: ask America about crime without punishment and what ultimately that does to a society.”

• Separated will air on MSNBC in the US on 7 December and play at Bertha DocHouse in the UK from 10 January