Ender’s Game [Amazon link] is one of the most well-known military science fiction books of the twentieth century. As renowned as it is, I think the short story version of it that Card wrote first is even better. As I feel like the short story is an important but underappreciated format at present, I’m going to encourage you to go find it and read it.
I read it in Jerry Pournelle’s There Will Be War Volume 1 [Amazon link], which is worth getting in any case if you don’t already have it. I own all ten volumes of this collection of military themed short stories and essays, and it is a favorite of mine.