Not even Fallujah, I think--more chaotic than that. In Fallujah, you had "standard" asymmetric warfare, with a relatively normal professional army on one side, and guerilla fighters on the other. I think it's unlikely in the extreme that we'll see anything like that in the United States. That is, I think it's unlikely in the extreme that we're going to have a Red Dawn type situation, where ordinary Americans are resisting an army (ours or anyone else's). I really think the model we should have in our heads is the most violent days of the George Floyd protests over the summer: loosely knit coalitions of civilians fighting each other, with the organized police and military sort of taking advantage of the chaos to fight everyone. A genuine civil war would deepen existing divides, and may even divide the cops and the military. It's not going to be the house-to-house fighting of Fallujah with a disciplined, professional group on one side and civilians on the other. It's mostly going to be civilian vs. civilian, with the "professionals" caught in the middle.