In a tantrum last week, President Trump raged against Kirstjen Nielsen, the homeland security secretary, in part because she had not done enough to break up families who crossed the southern border illegally. Mr. Trump thinks such callousness will deter families from illegally crossing over from Mexico, often to seek asylum from gangs and political violence.

Ms. Nielsen apparently took offense at Mr. Trump’s abuse, but her department is still in line with his “security-focused agenda,” making plans to use military bases to hold children separated from their parents by the Border Patrol, according to The Washington Post.

That is likely to become necessary since Attorney General Jeff Sessions announced that his prosecutors will file criminal charges against anyone crossing the border without authorization, rather than releasing them to await deportation. That legal escalation will tear apart more families with children, which now constitute 40 percent of people detained by American border agents, Ms. Nielsen told the Senate Homeland Security Committee on Tuesday.

That families constitute so great a proportion of illegal border crossers undercuts Mr. Trump’s contention that he is cracking down on immigration to keep out rapists and criminals. “These aren’t people, these are animals,” he snarled on Wednesday, in a stunningly dehumanizing reference to deported gang members. But his administration’s policies are not calculated to deal with the specific threat of violence, but rather to immiserate people whose only crime is wanting safety or opportunity. Migration north to the United States is largely what it has always been, an act of desperation by those fleeing violence, and of hope by those seeking new opportunity.