FAMILIES AND unaccompanied children detained at the Mexican border are often fleeing horrific conditions in Central American countries, especially El Salvador, Guatemala and Honduras, where violent gangs, drug trafficking and rampant criminality contribute to some of the world's highest murder rates. Now the Trump administration, alarmed at the recent surge in border crossers, is considering a new strategy to deter them. The message: "You think your native country is cruel? America is even crueller."

That's the logic behind a proposal under consideration by Homeland Security Secretary Kirstjen Nielsen that would try to discourage migrant families from crossing the border by threatening to separate parents from their children when they are taken into custody in the United States.

Until now, that approach has been beyond the pale for U.S. officials, who rejected it as inhumane and coldhearted in the extreme, given the trauma it would inflict on children, who by definition are innocent.

If Ms. Nielsen gives the green light to break up migrant families, many of whom have plausible asylum claims, she would be responsible for a policy whose heartlessness would rival that of Executive Order 9066, which authorized the forcible internment of some 110,000 U.S. citizens and noncitizens of Japanese descent during World War II. Four decades after that act of mass inhumanity, President Ronald Reagan signed legislation formally apologizing for it.

Arrests by the Border Patrol plummeted after Mr. Trump took office nearly a year ago, reflecting a decline in illegal border crossing driven at least partly by the president's aggressive anti-immigrant rhetoric. Despite that, detentions began climbing again in the spring — mainly of families and solo children. And in November, more than 7,000 "family units" were taken into custody at the border, a 45 percent surge compared with October; in the same month, the number of unaccompanied minors crossing the border shot up by a quarter.

U.S. officials are correct that those families take tremendous risks, often at the hands of coldblooded smugglers who guide them north to the border. They are also justified in wanting to discourage migrants from undertaking the journey, in which ransom, rape and other forms of abuse are rampant.

The right way to do that is not to double down on the cruelty with which those families already contend by tearing children from their parents' arms. What's more, it is unlikely to work in the case of families and children who flee their native countries in fear for their lives.

Heedless of horrendous conditions in Central America, the Trump administration cynically believes border-crossing families are trying to game America's system, with its years-long backlog in immigration courts and legal protections that allow many people to live and work freely while they await adjudication of their cases. In fact, many have legitimate asylum claims based on the threats they face in their home countries, and all are entitled to due process.

The idea of wrenching children from their families was first entertained in March by then-Homeland Security Secretary John F. Kelly, now the White House chief of staff, who said the minors would be "well cared for as we deal with their parents." Has a U.S. official ever issued a more chilling "assurance"?