I already said it: Taiwan.
Taiwan is a maritime fight. Not a land war. Therefore it'll be a naval and aerial conflict, and guess what, South Korea and Japan are both fairly competent naval and air powers. Both host US bases, as well, which means that if China starts bombing them directly, it'll trigger a war with the US.
I'm saying - this is the way they intend to engage - South Korea and Japan sail to Taiwan's defense on the front lines, while the US sits back and plays support. China is forced to fight those navies and air forces with one hand tied behind its back because if it starts bombing their bases, that triggers an all-out war vs. the US. So either China bites the bullet and goes all-out (low/moderate chance of victory) against the entire alliance, or (ideally for the US) it tries to do what Russia is doing in Ukraine, which is fight with one hand tied behind its back while the US supports from a distance. The latter may seem an "easier" fight but it is ultimately even more draining and beneficial to the US.