The video may seem too simple, too understated to mark a serious international incident in the South China Sea: a quick clip of a diver using a knife to cut a section of rope underwater. But that diver was with the Philippine Coast Guard, and the rope was part of a sea barrier placed by Chinese forces to keep Philippine boats away from an area they had a legal right to fish in. In that moment, the Philippines took one of the most forceful steps yet in contesting China’s unrelenting…