Straddling an international border, the twin cities of Nogales, Arizona, and Nogales, Sonora, are in many ways one community. For years the border was less distinct, with Mexicans crossing one way to visit family and friends and tourists crossing the other to roam the curio shops. But as times change, so do places like Nogales. The maquiladora industry has brought jobs, population growth, and environmental degradation to the Mexican side. A crackdown against undocumented immigrants has brought hundreds of Border Patrol agents and a 14-foot-tall steel wall to the U.S. side. Drug...