A good one to watch is Civil War with Kirsten Dunst. It is about what a real civil war in modern day America would look like. It is not an action film, but has some very intense action scenes, and the final battle over Washington D.C. is super intense. I've read some say it has the best portrayal of modern soldering tactics put to film.
The film itself is about a group of photojournalists trying to make it to D.C. to try and interview the President (now a dictator) before the rebel forces reach it. There are no political statements in the film (like no exposition as to what caused the war) which ticked a lot of people off because so many want the film to say "the other side" are fascists. The rebels are an alliance of Texas and California (with presumably other states allied) called the Western Forces (you can see how the filmmakers sought to neutralize any trying to read modern politics into it!).
A theme of the film, in addition to showing how bad a civil war would be, is also a critique of the photojournalism profession, as the journalists seem more caught up in who can get the best picture as opposed to really caring about the war itself.
It is an upsetting film though. Very brutal.