Look there is no way a splinter subgroup w/o modern military weapons is going to take on the US military, in a batallion against batallion type war. Claiming so, shows a profound misunderstanding of these weapons.

still some kind of guerilla style action where the combatants remain hidden within the population... that might be another matter. It might become practically difficult to contain such a threat w/o just killing all civilians -- which hopefully remain unacceptable.

your comparison to Vietnam is laughably inaccurate. we did not control those cities, and the ability to move weapons... none of this would be true in a civil war.

(Unless some part of our armed forced sided with the other side... THAT would be a bloodbath)