Presumably in a war where the US government fought its citizens there would be world powers that would support the citizens, whether they wanted to help the citizens or simply weaken the US government.
If they have serious backing why aren't they lobbing ATGMs at each other the way we see nation state aided militias doing in every other conflict this century (Syria is a great example)?
The Taliban is "backed" by external powers to about the same extent the IRA was.
The VC were conveniently for the North used up in the Tet Offensive, after that it almost entirely the regular forces of the North. Who didn't need planes or warships for their second huge armored invasion after support US was withdrawn for the South.