The Spratly islands are claimed by many countries including China (and Taiwan), Vietnam, the Philippines and Indonesia. See this article for more about the islands.
Although there are no indigenous inhabitants countries who claim the island at times send military personnel to assert their territorial claims. There are rich fishing grounds in the area and at times there have been confrontations between fisherman from one nation and military patrols of another nation that claims the islands. This has happened between Vietnamese fishermen and Chinese military vessels and more recently Chinese fishermen and Philippine military. NOTE: The fishing is damaging the environment.. Explosives are sometimes used damaging the reefs and other illegal methods are used as well including poisoning.
As well as the fish there is evidence of rich gas and oil deposits under the sea in the area. This ramps up the race to claim possession of the islands. The U.S. comes into the picture as a country that wants to extend its domain and project its power into the region. The U.S. also wants to confront and contain the expanding Chinese influence in the region.
Historically the U.S. has projected its power through defense agreements with allies in the region. After the second world war the U.S. had great influence in the Philippines after liberating the islands from the Japanese--along with Filipinos themselves of course. In 1951 the U.S. signed a mutual defense treaty with the Philippines.
The treaty states: “Each Party recognizes that an armed attack in the Pacific Area on either of the Parties would be dangerous to its own peace and safety and declares that it would act to meet the common dangers in accordance with its constitutional processes.” The U.S.supports the claim of the Philippines to the Spratly islands. Should there be a confrontation between the Philippines and China over the issue as there very well could be then the U.S. would be obliged to intervene on the side of the Philippines.
Recently the Philippines Defense Secretary Voltaire Gazmin claimed that the U.S. had an obligation to protect militarily Philippine territorial claims in the South China Sea. This would include the Spratly's. Gazmin pointed out that the U.S. Secretary of Defens Leon Panetta and Hillary Clinton secretary of state had both renewed their pledge that the U.S. will honor the 1951 military pact. China naturally has recently objected to the U.S. backing the Philippines and also military presence in the area.
This U.S. Philippine relationship could spark serious confrontations between the U.S. and China. For example Philippine warships recently tried to capture Chinese fishing vessels in the region but were driven off by Chinese fighting vessels. China is increasing its military presence in the area to counter Philippine actions. For its part the U.S. has increased its military presence and cooperation with the Philippine government. For more see this article.