So, in order to understand why the US would invade China, you would have to ask, what natural resources does China have that US corporations can plunder. Oil? No. Lithium? The US has more. Rare earth elements? Maybe. The reality is, the only thing that China really has more of than the US is people, and we have trouble taking care of our own. The US has about the same chances of invading China as I have of winning a billion dollars in the lottery. And those chances are nil, as I look at the lottery as a supplemental tax for people who are bad at math.
With that said, I think the worry from the standpoint of Marine readiness to engage Chinese troops is more based on supporting our allies in that region. While I pray every day that it does not come to this, I feel like it is inevitable that eventually China will keep taking countries and territories in the seas and along their borders until someone stops them. And unfortunately, I think that the US, despite going through its "Fall" phase of the rise and fall of our empire, is still the only political power on Earth that can stop them.