In a word, WOW. This is the most intense war movie I've ever seen. It makes Apocalypse Now! and Hamburger Hill seem like garden parties. The theme, however, is the ethical dilemmas of embedded photojournalists. When the unit you're in starts committing war crimes, do you intervene or just snap the photo?
The civil war itself may have been ripped from next year's headlines. It wouldn't take much for our current angry standoff to escalate into war as envisioned by Garland.
Garland has done for war movies what he did for sci-fi in Ex Machina and Annihilation. That is, he set the bar high.
On second viewing, there are 2 civil wars going on in Civil War. One is the Western Alliance vs. a President grabbing a third term of office. The other is white nationalist paramilitaries killing whoever they don't consider "100% American," that is white protestants. The latter has been going on for centuries.