To the American public it's a 2,000-mile-long project to keep illegal immigrants, narcotics, and terrorists on the other side of the U.S.-Mexico border. In the deserts of Arizona, it's a "virtual fence" of high-tech electronic sensors, cameras, and radar. In some border stretches it's a huge concrete-and-steel wall; in others it's a series of solitary posts designed to stop drug runners; in still others it's rusted barbed-wire cattle fences. For two-thirds...