The difficulty with that scenario is that the US is bound by two oceans and has a navy more powerful in some estimates than the rest of the navies in the world combined. Ukraine can be supplied because they’re contiguous with Western Europe. North Korea could be supplied by China, as could Vietnam. To supply the neo-confederates, Russia or China would have to cross an ocean and get past the US Navy, as well as the navies of other allied countries. Then they’d have to bring in the systems via either Mexico or Canada, both of which would be allied with the US.

I think you could imagine a scenario where they smuggle in small arms, but not artillery or other modern weapons systems.