THE TRUMP ADMINISTRATION ripped more than 2,600 migrant children from their parents’ arms with no plan or procedures for reuniting them, resulting in some 500 children remaining effectively orphaned even today, five months after the fact. Now it proposes a new policy for jailing migrant children indefinitely, one that ensures they “are treated with dignity, respect and special concern for their particular vulnerability as minors.”

That assurance, along with its rich irony, is offered by Homeland Security Secretary Kirstjen Nielsen, who has proposed the policy in a brazen attempt to escape the strictures of a two-decade-old court settlement forbidding the long-term incarceration of minors who cross the border seeking asylum in the United States.

Ms. Nielsen, who was instrumental in executing the zero-compassion policy that traumatized so many toddlers, grade-schoolers, tweens and teens this spring and summer, now would have Americans believe her department recognizes children as particularly vulnerable human beings, deserving of dignity and respect. How will that dignity and respect be meted out when those children are confined, along with their parents, in long-term detention facilities that the administration now proposes to build?

Ms. Nielsen, along with immigration hard-liners such as White House adviser Stephen Miller, are convinced that so-called catch-and-release policies are largely to blame for the flow of families across the southern border. Among the factors contributing to those policies is the 1997 court agreement known as Flores, which arose from abundant evidence that migrant children had been harmed by long-term detention, and forbade it.

The reality is that Flores has been in effect for more than 20 years, during which migrant flows have dipped and surged. When the Trump administration tried, just a few months ago, to amend the Flores agreement to permit long-term detention of families, U.S. District Judge Dolly M. Gee rejected its argument that the agreement was to blame for a recent surge in border crossings. “Any number of other factors could have caused the increase in illegal border crossings, including civil strife, economic degradation, and fear of death in the migrants’ home countries,” the judge wrote.

The administration’s proposal sets up a new court fight, one that will test Homeland Security’s risible insistence that the new policy would “satisfy the basic purpose” of the Flores agreement while freeing the government to get tougher on migrants. The “basic purpose” of Flores was to protect children from harm; confining them defeats that mandate.

It is legitimate to take concrete steps to ensure that migrant families appear in immigration court when ordered to do so. Ankle bracelet monitors, bail and other means of achieving that have been effective, and their use can be expanded. What’s less effective, and at odds with American values, is the administration’s abiding faith in punitive measures where children are concerned. There’s no evidence that they work to cut illegal border-crossing; there’s plenty of evidence of their cruelty.

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