Near the top of the list of ways that President Trump’s administration has deviated from established practice was his endorsement of a zero-tolerance policy for families seeking to migrate to the United States.

During the second year of his term in office, families hoping to escape violence in Central America or seeking more economic opportunity began arriving at the U.S.-Mexico border in record numbers to seek asylum in the United States. Immigrant advocates began to notice that children were systematically being separated from their parents and detained in different facilities. The administration at first denied that there had been any formally instituted policy change leading to the separations, but it soon became apparent that these separations were meant to evoke precisely the terror that inevitably resulted. The goal was a simple, callous one: make parents choose between seeking asylum or keeping their kids.

Make parents fleeing violence and poverty risk losing their children.

Following public outcry, the administration folded, ending the policy. Over the two years that followed, attention was redirected to other ways in which Trump’s behavior as president broke with established norms. But, as NBC News reported this week, hundreds of the children separated under the policy still hadn’t been reunited with their parents. When the policy first came to light, there were dozens of stories about the panic and cruelty involved, audio recordings of parents wailing as their children were taken away. The quiet suffering of the kids still separated from their parents, though, flew under the radar.

During Thursday night’s presidential debate, moderator Kristen Welker asked Trump how he would ensure that those kids and their parents would be able to see each other once again. The question prompted one of the most heated exchanges between Trump and his opponent, former vice president Joe Biden — and spurred a blizzard of at-times contradictory excuses and redirections from the president.

“Children are brought here by coyotes and lots of bad people. Cartels,” Trump began. “And they’re brought here and they used to use them to get into our country.”

This is a standard bit of rhetoric for Trump but one, as Biden was quick to note, that is untrue. There are sporadic examples of people arriving at the border with children who aren’t their own, though those are usually more distant family members (aunts and uncles, for example) or family friends. Nearly all of the kids at issue were, instead, traveling with their parents, which was entirely the point of the policy.

After Trump riffed about the border wall for a while, Welker pressed him on her question: “How will you reunite these kids with their families, Mr. President?”

“Let me just tell you,” Trump replied. “They built cages” — referring to the administration of Barack Obama. “You know, they used to say I built the cages. And then they had a picture in a certain newspaper and it was a picture of these horrible cages and they said, look at these cages, President Trump built them. And then it was determined they were built in 2014. That was him. They built cages.”

It’s true that, in 2014, a surge in children arriving at the border prompted the Obama-Biden administration to build systems to house them while they were processed. That surge, in fact, spurred a massive backlash on the right, providing political energy that Trump leveraged the next year in announcing his presidential bid. But Trump’s conflation of “building the cages” with “using them to house kids taken from their parents” is obviously ludicrous, like excusing kidnapping a dozen people in your basement by arguing that you didn’t build your house.

Welker again asked if Trump’s administration had a plan to reunite the children with their parents, which Trump insisted they did.

But they do not. As the New York Times’s Caitlin Dickerson reported, the parents are being sought by court-appointed lawyers after the administration was forced to release information about the separated children. This is no easy task; as was made obvious at the outset of the policy, the separation policy was haphazard and sloppy, often excluding proper record-keeping which might allow reunification.

After Biden pointed out that Trump’s claim about the children being smuggled was false, Trump interjected to say that they — Obama and Biden — did it but he “changed the policy.” This is an old canard of Trump’s, pretending that his policy on children was simply a continuation of his predecessor, which it wasn’t, and that the policy he ended only under duress was, instead, his nobly ending a hated inheritance from Obama.

Trump began taunting Biden, asking the former vice president who built the cages. Biden, visibly angry, sidestepped the comments.

“Let’s talk about what we’re talking about,” he said. “What happened? Parents were ripped — their kids were ripped from their arms and separated and now they cannot find over 500 of sets of those parents and those kids are alone. Nowhere to go. Nowhere to go. It’s criminal, it’s criminal.”

Trump had a rebuttal.

“Kristen, I will say this: they went down, we brought reporters, everything,” he said, referring to a belated tour of one facility that occurred as the policy was in place. “They are so well taken care of. They’re in facilities that were so clean, that have gotten such good —"

Welker jumped in before Trump could finish whatever point he was going to make.

One of Trump’s rhetorical tics is that he tends to land in well-worn grooves. This was one: As the policy was being debated, his team tried to defend it by suggesting that the children were being well cared for. But, of course, that’s not really the broader issue that was being debated. This was, after all, the president who a few minutes prior had referred to the cages in which the children were being housed as “horrible”; now, though, looking for a point to score, he slipped into his “we took good care of the kids who we took away from the families they loved and with whom they’d traveled thousands of miles seeking a safer life.”

Regardless, even that claim wasn’t true. The conditions in which they were held were often dire and dirty.

That this policy has largely been absent from the conversation over the course of this election is, if nothing else, a sign of the breadth of concerns Trump’s critics have raised about his tenure in the White House. It seems likely, though, that the family separation policy will be one of the first decisions included in future summaries of Trump’s presidency, whether it lasts for one term or two.

It proved so indefensible in 2018 that Trump walked away from it, something he’s only rarely done. Two years later, seeking four more years in office, Trump was unable to effectively defend it.