ACUNA, Mexico—The singular focus is palpable. Once migrants leave the bus station in the Mexican border city of Acuña, it’s a brief taxi ride or walk to the banks of the Rio Grande and to their ultimate goal—the United States. The taxi driver charges 50 pesos for the three-minute drive, double the regular fare, but no one quibbles. A rocky but well-worn path leads down to the shore, and the migrants walk briskly and with purpose. Many, especially the Haitians, pause to tear up and discard the paperwork they have with them—passports, visas, and identification cards are strewn all over the ground. They’re told it’s harder to be deported from the United States if they don’t bring ID. Two elderly Mexican men are raking through the trash to find anything of value. To them, the passports are trash. They end up among the piles to be burned, as evidenced by …