There's certainly other forces at play, but having participated in one, I tend to view the main reason that war is traumatic to the individual is its depersonal

There's certainly other forces at play, but having participated in one, I tend to view the main reason that war is traumatic to the individual is its depersonalized (and in modern times, mechanistic) nature. A person being confronted so undeniably with their place in the machinery of superorganisms in conflict induces all sorts of problems, including cognitive dissonance, and the mind struggles to form the individually senseless nature of strangers lining up to disassemble each other with kinetic force into some kind of cohesive narrative, usually failing miserably. So, that's the true horror of it: the deep destruction of the self. That's why what remains of participants are often these psychological husks, their reality model permanently shattered.