Let's be real about this. Just because you see a movie about something, doesn't mean you know anything about that thing. In my experience, there's only one way to know anything about anything and that's to experience the thing itself. The rest is just writing.
I've never been in the military -- I wouldn't be allowed -- and so obviously I've never been in a war. Whatever I say here does not have anything to do with what it's really like to be a United States Marine, nor does it have anything to do with what it's like to be in a war.
Our "Jarhead" hero, Swofford, is a 20 year-old who in 1989 for some reason we aren't told decides to become a Marine instead of going to college. After training, our boy is selected to be a sniper and he thereupon learns that snipers, unlike the Lone Gunmen we all want to believe in, work in teams of two. One member of the team is called the spy and the other is called the shooter. The spy locates the target, supplies range and wind speed information to the shooter, then when all the pieces of the shot are in place, the spy says slowly: "Fire... fire... fire."
On the third "fire", the shooter pulls the trigger.
There's a famous Hollywood bio-pic about Vincent van Gogh called "Lust for Life". Heh. Movies about artists. They generally suck, in my view, though that is not entirely the fault of the filmmakers. We have so much crap in our heads about what it means to be an artist. If I were tasked to make a movie about an artist, I would leave everything about him being an artist out of it. A guy's art is pollution when it comes to considering his life.
Or, if I were tasked to make a movie about an artist but not about any artist in particular, I would make a movie about an artist who works in a medium we wouldn't normally think of when we think of art. For example, the art of shooting from a great distance your country's enemies in the head.
"Lust for Life" is about an artist who ultimately sees himself as a failure. He cuts an ear off and eventually kills himself, after all. "Jarhead" is similarly about an artist who fails to find fulfillment in his work, but there is a difference in the manner in which these two artists see themselves as failures, of course, and this difference is accounted for thusly: van Gogh actually gets to paint some pictures; Swofford never once gets to shoot anybody in the head.
"Lust for Life" is Hollywood bullshit, essentially unwatchable on account of first-degree shlock. "Jarhead", however, is the best account I've ever seen of what it feels like to fail in your art.