Personally? China. Is it rational, IDK. It's just the largest more capable power at this moment. (particularly their industrial base and ability to out produce weapons / soldiers)
Maybe Xi decides they're mad that we're defending Taiwain.
Maybe the decide to do the inverse vietnam and that the American left deserves to be freed from "Trump" and that communism is freedom and virtuous to give to the American people.
Like I said, probably not rational.
They don't want to govern California; they want to ensure that the global infrastructure, shipping lanes, and institutions run on Chinese technology and standards. From Beijing's perspective, a kinetic war on the US mainland destroys the exact global economy they are trying to lead.