The South China Sea dispute explained. Just one explanation
https://www.youtube.com/watch?time_continue=5&v=8eqtl0ym1p8&feature=emb_logo&fbclid=IwAR0KfuZYJQKhN1-iBBR-YHv7LCdfZyWRM3M6EqmQ_CeeZT5gi4fra1iuqr0
U.S.-Republic of China (Taiwan) Mutual Defense Treaty
https://en.wikipedia.org/wiki/Sino-American_Mutual_Defense_Treaty
> Treaty
When Taiwan peacefully became a democracy (at the end of authoritarian rule by the Nationalists) it became very hard for the USA to just abandon Taiwan to the loving hands of the Communist mainland.
Second, my understanding, is that the population of Taiwan is about 50% indigenous Taiwanese and 50% descendants of Nationalist Chinese who fled the mainland in 1949. The indigenous Taiwanese want nothing to do with mainland Red China and it's the descendants of the Nationalists that are more ambivalent about reuniting with the mainland. This may have changed after China's heavy hand in Hong Kong.
"China's heavy hand in Hong Kong"
How China's heavy steps in Hong Kong reverberate in Taiwan
"No one anticipates any steps threatening Taiwan's autonomy on the order of the mainland's tightening grip on Hong Kong any time soon. Beijing would seem to have its hands full managing events in Hong Kong and other restive regions like Xinjiang and Tibet, economic fallout from the coronavirus, the U.S.-China trade spat, and steering China's rise to dominance in a dynamic Asia."
https://www.csmonitor.com/USA/Foreign-Policy/2020/0612/How-China-s-heavy-steps-in-Hong-Kong-reverberate-in-Taiwan
China fires missiles in South China Sea 'in warning to US' after plane entered no-fly zone
https://www.express.co.uk/news/world/1327725/world-war-3-china-missiles-US-south-china-sea-latest-donald-trump-war