I just did this topic for a final paper, so let me tell you all a bit of what I found in my research.
Let's put gas and oil exploitation aside for a second and just look at some territorial claims. In the early 20th century Chinese naval mapmakers included the entire South China Sea as their territory. This is the first written recording of the historical claims, and the Spratlys, Paracels, etc. were under the governance of Guangdong province (later transferred to Hainan province). However, the French claimed many of the islands as well and after WW2, many French and ROC troops were on the islands. Now in come the PRC.
The PRC officially claimed the area in a declaration in 1958. Throughout the 60s, China forcibly took back most of the ROC/Taiwanese controlled islands, and all French claims were transferred to Vietnamese control. Where Vietnam's claims come from is the 1951 San Francisco post-war conference, where they claimed the lands and the delegation unanimously voted to grant them the claims, even though PRC wasn't represented there. The Filipino claims are mostly fabricated, stemming back to only 1976 or so, though a Filipino explorer had privately claimed the islands for himself two decades earlier.
As South Vietnam collapsed, the PRC invaded a few of their islands and took them before a unified Vietnam could defend them. China later had a naval battle in the late 80s with Vietnam over some of the same islands.
So that history aside, the USA and Japan will probably only step in if there is a threat to international trade. They haven't stepped in before, and they probably won't step in in the future.