By Robert A. Heinlein.
This is a lovely coming-of-age novel, with some war story and a fair amount of political philosophy thrown in for good measure. I just love it. I learned about balancing authority with responsibility from this book, and stopped taking universal suffrage for granted. I started wondering if "juvenile delinquent" was a valid concept, and wondered if it was really right to essentially ignore offenses committed before ones 18th birthday, and punish harshly ones committed after. Mind you, I don't now and never did claim that Heinlein had the right answers to any of these questions; but he made me see many of them as questions for the first time.
The movie, based on the trailers and friends' reports, is highly regrettable. And the current paperback of the book has a movie tie-in cover. Ugh.
Oh, and incidentally, he clearly did mean specifically military service.