It's a moonless night, and Jorge Macias Onate, a Mexican border-police officer, knows he'll be busy as he eases his Dodge Ram along his country's porous border with the U.S. Soon his headlights pick out four scraggly youths preparing to scale the 12-ft.-high steel fence that separates the town of Agua Prieta from Douglas, Ariz. As he slows the pickup, the teenagers scatter like rabbits toward the sagebrush. "Wait! Don't run! We're not here to arrest you," yells Macias. "We want to help you. The problem is on the other side."
Macias and two colleagues are members of Grupo Beta, a...