NUEVO ORIZABA, Mexico -- Transnational drug cartels are carving out new sanctuaries along this wild, neglected frontier with Guatemala, muscling their way into Central America’s weak states and moving freely over uncontrolled border crossings that do not appear on any map. More than 90 percent of the South American cocaine that reaches the United States now funnels through Guatemala and across Mexico’s southern border, according to the latest intelligence from the U.S.