A priest who shelters stranded migrants needs police protection, a chopped-up body turns up with a threatening message, beheadings are on the rise, and the local press is too frightened to write about any of it. This is not northern Mexico, where traffickers created by drug prohibition fight for turf along the U.S. border and the Mexican government wages an open battle against them. This is the south, where the brutal Zetas organization is quietly spreading a reign of terror virtually unchallenged, all the way to the border with Guatemala — and across it.