I went and saw A24's Civil War (spoilers topic)

There are like 4 lines in the film about the cause of the war. You can infer but I think them tying Cali and Texas together blurred it so much you don't actually know what the war is over. Basically the behind the scenes is the president took over a 3rd term and it started the war. It doesn't say almost anything else about it.

Yeah the ending was a bit loose but I imagine at that point of loss there's no one left to fight for the president.