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.

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The uninvited guest: Chinese sub pops up in middle of U.S. Navy exercise, leaving military chiefs red-faced | Mail Online