Brian, a U.S. Border Patrol agent who works along the south Texas border, is haunted by something that happened a few years ago. A man—a Mexican cartel member, he believes—emerged from the banks of the Rio Grande carrying two toddlers. The children, a boy and a girl, were wearing nothing but diapers. The man darted across the border, dropped the children fifty feet away, and then raced back into the river to Mexico. 

“I picked up these toddlers and looked fifty yards south,” said Brian, a ten–year veteran of the agency, who, like all agents we spoke to for this story, insisted on using a pseudonym. That’s when he saw six adult migrants running across the border as fast as they could. The children, he realized, had…