NUEVO LAREDO, Mexico - The ghosts of the world's most infamous, powerful and ruthless drug lords have risen violently in this turbulent city along the South Texas border. For years in cities such as Medellin, Colombia, and Tijuana, Mexico, men such as Pablo Escobar and the Arellano-Felix brothers used murder, kidnapping and extortion to terrorize law officers and enemies alike. Those men are now gone, killed or in custody. But a new generation has emerged, and this one has set its sights on Nuevo Laredo, a city of 500,000 people across the Rio Grande from the fastest-growing U.S. border city,...