As I understand it, an enormous percentage of global trade sails through the South China Sea. From a pragmatic standpoint, it behooves American interests to support the Vietnamese and Philippine territorial claims so that one nation (China) does not militarily and/or politically dominate such a vital trade route.

From an idealistic standpoint, America supports the notions of state sovereignty and the U.N. Convention on the Law of the Sea simply out of principle. Like Apple’s stance on customer data privacy, the U.S. government defends freedom as a matter of pragmatism first, and then talks up the altruistic angle :)