Why the relative power of the aggressor doesn’t matter in the beginning (see Russia vs. US). Qui audet adipiscitur, yo.
I really don’t think we should get involved in a land war in Asia. If it comes down to actual people in combat, I’m pretty sure it’s NATO’s turn to step up and by NATO I mean not the US and some token Western European troops, but Western European troops and some US support. It’s their backyard, after all.