The U.S. set off China’s ire in 2010 when Secretary of State Hillary Clinton, speaking at a regional summit in Hanoi, called resolving the competing claims to the sea “a leading diplomatic priority.” That drew a rebuke from Chinese Foreign Minister Yang Jiechi, who said internationalizing the incident with U.S. involvement “can only make matters worse and more difficult to solve.”
-Bloomberg
Basically, in response to a US military spending spree in region, the China has claimed Southeast Asia as its bitches.