There have certainly been times in the last few unnecessarily-crazy weeks when I worried US Republicans would impose capital controls [0] like China does. Partly to affect how dollars are being used (or not used) and partly to prevent Americans from escaping with more than whatever they can carry through a border checkpoint.

Just one among a bunch of things that would be unthinkable a decade ago.

[0] https://en.wikipedia.org/wiki/Capital_control

We already have it to some degree, it's not easy to own foreign currency in the US and for the most part it must be bought in the form of "futures." Foreign banks have special reporting requirements for US citizens and many foreign banks will absolutely not take US money without large minimum balances or a local job.