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.

I don't necessarily see that happening. No other country came in on the side of the Confederacy during the US Civil War.

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.

Taliban never had the backing of Russia, what is this bs you're spreading?

The NY Times spread some misinformation that Russia had offered a bounty to the Taliban for killing American soldiers [1].

[1] https://www.nbcnews.com/politics/national-security/remember-...

You just shared a article saying the had contacts with them. South Korea has a contact with North as well, that is how countries operate.

Who cares if they are backed. Are they fighting with small arms or not? The answer is yes.

Did VC and Taliban have tanks, planes, or warships?

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 Who didn't need planes or warships for their second huge armored invasion after support US was withdrawn for the South.