Gunmen ambushed at least a dozen police officers and a local security supervisor on Monday in a coastal town near Acapulco, once a jewel of Mexico’s tourism industry. Criminal gangs in the state of Guerrero are increasingly taking on Mexican authorities as they fight turf battles over the region’s heroin trade. The World’s Marco Werman speaks with Falko Ernst, a senior researcher for Mexico at the International Crisis Group, about what’s behind the rise in violence.