It's Memorial Day, and that puts me in mind of many things, including military sf, and how different authors have such wildly different views on war. I realize that Starship Troopers is a classic, but (confession time here), I've tried reading Heinlein, and I just can't get into his stuff. I've seen the movie, which I'm sure is only somewhat like the book, and so I offer no opinion on the theme of war in that particular story.
I have, however, read The Forever War, by Joe Haldeman, who, incidentally, served in the military during the Vietnam War. I found this book to be a criticism of the Vietnam War, the poor reception the troops received when they returned home, and how difficult it can be for a soldier to assimilate to non-military society once their service is over.
Orson Scott Card's novel Ender's Game is finally coming out as a movie later this year. I've read this one twice, and enjoyed it. If you haven't read it, do so before the movie comes out, just in case the movie screws up the ending. In this novel, the enemy is completely and utterly alien, which I read as a metaphor for how people often view the enemy. They look different. They have strange customs. They're non-human, other. These are the sort of messages people often encourage in order to make it easier to kill the enemy. This novel is more of a commentary on the lens through which we view the enemy.
Most recently I read Old Man's War by John Scalzi. As the title suggests, the elderly are sent to a faraway front line in order to fight the enemy. Soldiers are often seen as expendable, and so are the elderly. I found it an interesting line to draw between the two groups, and the premise certainly held my attention. My only quibble with this novel was that I never felt all that drawn to the main character. He often came across as cold and aloof, even though I know he wasn't, and so I don't plan to read the rest in this series.
Military sf shows the effects of war through the lens of something new and different. It's a subgenre worth reading.