"Why has China's foreign policy been more aggressive over the past three years? Why have the Chinese undone a decade's worth of good neighborly diplomacy in Asia? The facts of the case are now well known. In Southeast Asia, China declared the South China Sea a "core interest" (a term previously reserved for Taiwan and Tibet), in essence defining the sea as Chinese territory. To punctuate the point, China harassed Vietnamese fisherman near disputed islands in the sea." (Quoted from article)