The Chinese economy would die a quick death if they went to war against America and her allies over these islands/economic zone. They’d lose a war, plus lose Korea, Japan, Vietnam and the entire area as trading partners.
The history of SE Asia isn’t one of trust in China as a fair dealer. They fear a Chinese hegemony. China’s on the cusp of becoming an also-ran. You’ve got the equivalent of Cook County (Obama Country) trying to maintain control on a population that wants to be Hong Kong. They’re going to lose, but it can be smooth or bloody.
The Chinese seek to increase their control over what are likely vast oil and gas reserves offshore. If people have been paying attention, the Chinese have been buying up mineral rights/resources worldwide, and not just oil and gas, but coal and base and precious metals as well.