In an astonishing work of scholarship that reads like an adventure thriller, historian Buddy Levy records the last days of the Aztec empire and the two men at the center of an epic clash of cultures.

Ciudad Juárez lies just across the Rio Grande from El Paso, Texas. A once-thriving border town, it now resembles a failed state. Infamously known as the place where women disappear, its murder rate exceeds that of Baghdad or Moga...