I concur with Cacophonix and greatly appreciate his very thoughtful analysis of the situation. Just to add to his points, US and its allies consider China, like somebody mentioned above, as a legit threat that has started building up its world dominance economically, militarily and hence polictically lately. So, its silence about the incidence is a part of its Check and Balance strategy.

Further more, one cannot simply overlook the real reason behind USA's involvement in the Vietnam war, COMMUNISM !!