Alejandro Salazar slumps into a cushioned seat halfway into the bus, looks out from under a baseball cap and thinks about a future more than a thousand miles away. He came to the Las Vegas Valley from Mexico five years ago to build houses, launching a run of $700-a-week paychecks that made it worth crossing the desert into the United States. But those weekly checks were whittled down to $200 over the past year; he sold his van and is returning to Aguascalientes. “At least I have family there,” he says, shrugging his shoulders. He has $60 in his jeans...