Our navy would utterly destroy theirs in open combat. Their only hope would to surprise attack us with thousands of anti ship missiles, and make us leave the region. With a couple thousand marines, combined with a coalition from the dozens of surrounding countries that hate China, we would have a very good chance of defeating them in the South China sea, or near the Japanese islands. You don’t need a massive force of marines to take an island, just look at ww2, we would storm islands with around 30,000 marines.
China’s only experience in warfare within the past 40 years has been getting their asses handed to them by Vietnamese militias.