When I was on my basic training to become an army chaplain, one of the books that was highly recommended was On Killing by Dave Grossman. Grossman is not a chaplain and doesn’t speak much about religion, but this should be required reading for all military chaplains.
In addition to being an experienced officer, he also has a strong background in psychology. Grossman has looked, from both a psychological and historical perspective, what it means when we ask human beings to kill fellow humans.
Movies and television shows would suggest that there is very little cost for one person to kill another. Aim and shoot, as simple as that.
Grossman demonstrates that there is a high emotional cost to killing, even in self-defence during a time of combat. It was bad enough when there we two standing armies. It is more complicated now that enemies are often indistinguishable from civillians.
This was a powerful book that really challenged my presuppositions. I was surprised by the number of soldiers who actively avoided killing during war and what militaries did to overcome that reluctance.
If you are interested in military history or even just the human condition, I would highly recommend reading On Killing. Even if you never consider serving in such a way, this book is an important counter-narrative to the message of popular media.