Countries surrounding the South China Sea have built more than 90 military outposts in around 70 locations in the highly disputed area, at times turning semi-submerged reefs or plain rocks into inhabitable islands in an attempt to establish control over the territory. This is according satellite images analyzed by the Asia Maritime Transparency Initiative at the Center for Strategic & International Studies.
Oil reserves and fishing grounds make the waters of the South China Sea desirable and claims over islands (but not rocks and reefs) have been hoped to create rights to exploit these resources. Additionally, important shipping lanes crisscross the area and China equals control of them with regional and even global power. While returning President-elect Donald Trump has been known for its tough stance on the Asian superpower, he has also rejected military deployments abroad, making the issue of a potential escalation in the South China Sea more contentious for U.S. allies Taiwan and the Philippines.
The conflict around the South China Sea is centuries old but has recently been receiving lots of attention in connection with China-Taiwan relations. However, the idea that China could start a potential invasion of Taiwan by attacking its two outposts in the area is just a small part of a much bigger, multilateral conflict. China is usually more at loggerheads with other countries in the South China Sea, including ones that are usually considered its allies, like Vietnam. At the same time, Taiwan actually mirrors many of China's claims in the area as it historically sees itself as the rightful ruler over all of Chinese territory, thereby appearing as a defender as well as an aggressor in the region by laying claim to islands quite far removed from its territory, just as China does.
Despite its very murky origins on British naval maps, Chinese claims in the South China Sea are sweeping and extend to localities within other countries' Exclusive Economic Zones. For the Philippines, this includes Second Thomas Shoal, which it controls but China nevertheless claims, and Scarborough Shoal, which has been de-facto controlled by China by a continuous coast guard presence despite no structures having been built. China also lays claims to areas in the Western Spratlys that Vietnam considers its exclusive grounds and has disrupted the country's oil exploration and fisheries there. It has done the same in the Paracel Islands, which it controls since a military victory in the 1970s and where a 2014 standoff over China's attempt to install an oil drilling platform made international headlines. Both countries' shores are within 200 miles of that second island group. China has built installations on 20 features in the Paracels.
Vietnam has built on 21 features in the Spratly Islands—the largest number by far—and is feuding with China, who occupies seven built-up reefs there, over who has controlled the island group in the past. According to the BBC, it is Vietnam who is more likely to prove the above. In its feud with China over a portion of the Spratlys, the Philippines won a landmark victory in 2016 in international court, but the success hasn't had much real-life consequences. Malaysia, finally, is occupying five built locations in the Spratlys close to its territory, while Brunei also lays claim to some, but hasn't built up any.