I'm not entirely sure if this is also a reference to Taiwan. It's probably not a reference to only Taiwan, because it's long been U.S. policy - at least officially - to defend Taiwan from any prospective Chinese attack.
But the main issue is what to do about the fact that China has helped itself to spots in international waters in which it has built, for all intents and purposes, military installations. Trump's position - and also Tillerson's, as if that needed to be explained - is that China has no right to do that.
