When the U.S. Navy deploys a battle fleet on exercises, it takes the security of its aircraft carriers very seriously indeed. At least a dozen warships provide a physical guard while the technical wizardry of the world's only military superpower offers an invisible shield to detect and deter any intruders. That is the theory. Or, rather, was the theory. Uninvited guest: A Chinese Song Class submarine, like the one that sufaced by the U.S.S. Kitty Hawk American military chiefs have been left dumbstruck by an undetected Chinese submarine popping up at the heart of a recent Pacific exercise and close...