The Los Zetas Cartel changed the game in the Mexican War on Drugs. From the mid-2000s, they introduced an unprecedented level of violence – paramilitary-style executions, beheadings, bodies hung from bridges. But this was not simply psychopathic violence. These were special forces soldiers from the Mexican military – who had received specialized training from the US Army – who had defected, and used their training to become the most feared drugs cartel in Central America.