Twenty miles south of the Arizona-Mexico border, the rugged, beautiful Sierra Madre mountains begin their dramatic ascent. Almost 900 miles long, the range climbs to nearly 11,000 feet and boasts several canyons deeper than the Grand Canyon. The rules of law and society have never taken hold in the Sierra Madre, which is home to bandits, drug smugglers, Mormons, cave-dwelling Tarahumara Indians, opium farmers, cowboys, and other assorted outcasts. Outsiders are not welcome; drugs are the primary source of income; murder is all but a regional pastime. The Mexican army occasionally goes in to...