ABOARD A US MILITARY AIRCRAFT - General Martin Dempsey denounced ex-officers Tuesday for waging a campaign against President Barack Obama, arguing that soldiers had a duty to stay above the political fray.Wading into a potential minefield during a hotly-contested White House race, Dempsey voiced his disapproval of a group of retired military members and CIA officers who have accused Obama of spilling sensitive national security details to help secure his re-election in November.Asked if the group's criticism was valid or useful, Dempsey, chairman of the US Joint Chiefs of Staff, said he would not comment on the substance of their...