Its not really about an American civil war. There are civil wars going on all over the world we hear about them on the news, but we dont really think about them too much. I think the purpose of the movie is to force American viewers to consider what a civil war must look and feel like for people elsewhere in the world, and the filmmakers achieved that to great effect by setting the movie in the USA. The contrast between the very familiar sight of American locales with the very unfamiliar sight of military forces occupying those locales was unsettling.
For me, the reason for the civil war was completely irrelevant I didnt care about the backstory. The fact that one of the factions is California teaming up with Texas just felt to me like the filmmakers yelling THIS IS A FANTASY. The purpose was not to accurately recreate what an actual American civil war might look like, but rather to put the viewer on the ground in a civil war any civil war among all the chaos, and confusion. So what if the viewer doesnt know all the details of whats going on? Most of the citizens that the protagonists run into dont really seem to know whats happening either. For me, the lack of info was part of the disorienting effect of the movie, and it worked.
Yet despite not knowing what was really happening, I felt somehow satisfied by the ending. What happens at the end is very often the fate of evil dictators. And yet, I had to consider that I didnt really know the full story. What if the president wasnt really the bad guy? What if the Western Forces were a bunch of Nazis and the president had just failed to stop them? History really is written by the victors.
All in all it was a movie that stuck in my head for more than a few days, and not many movies do that.