But Mr. Miller has expressed none of the president’s misgivings. “No nation can have the policy that whole classes of people are immune from immigration law or enforcement,” he said during an interview in his West Wing office this past week. “It was a simple decision by the administration to have a zero tolerance policy for illegal entry, period. The message is that no one is exempt from immigration law.”

The administration’s critics are not buying that explanation. “This is not a zero tolerance policy, this is a zero humanity policy, and we can’t let it go on,” said Senator Jeff Merkley, Democrat of Oregon.

“Ripping children out of their parents’ arms to inflict harm on the child to influence the parents,” he added, “is unacceptable.”



Beyond those moral objections, Jeh C. Johnson, who as secretary of homeland security was the point man for the Obama administration’s own struggles with illegal immigration, argued that deterrence, in and of itself, is neither practical nor a long-term solution to the problem.

“I’ve seen this movie before, and I feel like what we are doing now, with the zero tolerance policy and separating parents and children for the purpose of deterrence, is banging our heads against the wall,” he said. “Whether it’s family detention, messaging about dangers of the journey, or messaging about separating families and zero tolerance, it’s always going to have at best a short-term reaction.”

And that view was based on hard experience.

When Central American migrants, including many unaccompanied children, began surging across the border in early 2014, Mr. Obama, the antithesis of his impulsive successor, had his own characteristic reaction: He formed a multiagency team at the White House to figure out what should be done.