Reminder that rare earths are not actually rare, it's just that China has 85% of world refining capacity for them as it takes some doing. In a full on war footing, pull out all the brakes type situation the US could absolutely spin up some not as efficient, very dangerous for workers, expensive, but workable emergency capacity, it wouldn't put them on par with China but it would be enough to continue a war with selective strikes (instead of constant, high intensity bombardments hourly for weeks on end) and after that point the US would almost certainly continue to build up that capacity and make it a point to attempt to wean the rest of the world off Chinese rare earths.

It would also result in all kinds of other unforeseen consequences. For example the US may have the capacity to hack and physically damage or destroy Chinese electrical or other infrastructure but obviously isn't going to do that right now. But if a hot war unfolds and especially one where the US isn't winning they might break out those offensive state malware toolboxes and start wreaking incredible havoc within China that could cost a lot of money, take a lot of lives and result in a lot of chaos, uncertainty and problems that would take quite some time to sort out. The more electronically connected and "smart" everything is as in China the easier it is to fuck up everything, ironically the US by not being as "advanced" as China in adopting network connectivity to various things is a harder target in this regard. China is well aware of this risk, the US basically compromised everything in China with their shipments of NSA hardware back-door chipped Cisco routers and devices which Snowden revealed and though it's been more than a decade since they became aware I have to assume they're still inside many systems lurking and waiting.