VERACRUZ, Mexico -- Five years after President Felipe Calderon launched an offensive against Mexico's five main drug cartels, the nation is now dominated by two powerful organizations that appear poised for a one-on-one battle to control drug markets and trafficking routes.The government's success in killing or arresting some cartel leaders has fractured most of the other gangs to such an extent that they have devolved into quarreling bands, or been forced to operate as subsidiaries of the two main cartels.