Homeland Security Secretary Kirstjen Nielsen, left, walks next to a section of the border wall fortified with razor wire separating Tijuana, Mexico, and San Diego, on Nov. 20. (Gregory Bull/AP)

IN TESTIMONY to Congress on Thursday, Homeland Security Secretary Kirstjen Nielsen spoke at length, and repeatedly, about the threats to which Central American migrants are subjected when they traverse Mexico on their way to seeking asylum in the United States. That was directly after she announced that asylum seekers who do reach the United States will now be returned immediately to Mexico, where they will await their scheduled court appearances — a process that currently takes more than three years.

Ms. Nielsen failed to explain why the migrants, whom she described as constantly beset by rapists, traffickers and other predators in Mexico, would fare better if forcibly returned to Mexico than they were while in transit there.

That’s the dangerous paradox, and the hypocrisy, at the heart of the Trump administration’s apparently unilateral move to compel Mexico to serve as a waiting room for tens of thousands of asylum seekers, most of whom now come from Honduras, El Salvador and Guatemala. Facing vicious criminal gangs and scant economic opportunity, they flee those countries in search of better lives in the United States.

It is true that the surge in family groups seeking asylum has overwhelmed the U.S. system, including shelters bursting at the seams and immigration courts where the backlog is approaching 1 million cases. The administration is correct to be concerned by what President Trump calls “catch-and-release” — the practice of permitting asylum seekers to remain in the country, often for years, while awaiting court hearings that many migrants will skip. That is a form of dysfunction, and one that may encourage Central Americans to attempt the dangerous journey.

The question is which form of deterrence is workable, humane and legal. Family separation, which the administration tried to disastrous effect last spring, failed on all three counts. A more recent gambit, slow-walking processing at legal border points of entry, has had little or no deterrent effect. For now, courts have blocked the administration’s attempts to narrow the criteria for asylum claims; and, on Friday, the Supreme Court quashed the administration’s attempt to require that asylum seekers cross only at official border points of entry.

It’s unclear whether the new “remain in Mexico” policy will dissuade migrants from making the northward trek, if U.S. courts even allow it to stand. (Forcing asylum seekers to wait in a third country may not be legal.) It also remains to be seen whether Washington’s new stance results in unintended consequences — for instance, a surge in illegal entry by migrants who may despair at the prospect, and perils, of drawn-out waits in Mexico.

What is clear is that the United States is not absolved of responsibility for migrants legitimately seeking asylum simply because they are compelled to wait elsewhere. If asylum seekers are preyed on, exploited and harmed after having been returned to Mexico, U.S. officials will not be able to shrug off their moral responsibility. The United States is bound by law and international obligations to welcome and vet migrants fleeing persecution, and to grant asylum to those who meet specific criteria. That obligation cannot be abrogated by an announcement.