You're completely overlooking the fact that fighting a foreign war =/= fighting a domestic threat. The Afghanis greatest asset is their ability to use their land/people to hide and strike. I don't think you get that advantage in a US civil war, because who would know the US better than US citizens.

Even the previous US civil war isn't a great example because that was a regional war before modern technology. You could have guerilla forces hiding out in southern backwaters that the Union wasn't familiar with. It would be much harder to do that in a country as massively surveilled as the modern US.

These wars would not be the same, and this represents a fundamental misunderstanding of the scope of the conflict.