It will indeed require a lot of content. You need to determine the bounds of the story and give the player freedom to do what they want inside those boundaries. The problem with writing for games is that the player can also be the story teller.
Like Conquestor said, having a few fleshed out paths is a good way of doing this. You can write the hero path, pacifist path, and murderous path, for example, and give options related to these paths. Of course, it's not without its problems. You can't force the player to follow through a single path in the whole game. They might start being a pacifist at first, but there's nothing stopping them for killing people for reasons like boredom or curiosity. What if they act like both a hero and a murderer, etc.
Another idea is to make consequences of the actions the player take appear immediately. If you choose to spare a soldier, a few scenes later you might see him killing other people, and that thing is done. You might not see the soldier again for the rest of the game (thus less content to make) and the player can rethink their future choices because now they understand that their choices matter. This way they might realize that their lack of courage to kill caused more people to die, and they might choose to be more ruthless next time, or the other way around.
I think that would be enough for now. Writing down all the possibilities would take forever. Just determine the boundaries of the experience. What kind of experience you want your players to feel? What is the message of the story? Is it about life in war? Agency? Once you've determined what your game is about, you can build around it.