I've been racking my brain all week trying to remember the name of this book so I can go back and finish it. It's been almost a decade, so I may remember some of the details wrong.
The setting seems to be in a future version of Earth where complex technology is outlawed with religious fervor. It hints at a near apocalyptic event that brought this about, but I didn't learn the details. The technology level is roughly early industrial. The vibe is almost steampunk vibe mixed with American military tradition and British aristocracy.
The book starts off with the main character in military school, eventually being deployed to Australia, I think. It talks a lot about his correspondence with his girlfriend through letters, and I think I remember something about a campaign in India.
The "someone"'s name is a male name that feels familiar because it's also the name of a prominent historical figure, and is the name of the main character's friend/rival at the beginning of the book.