This is one of the classic science fiction novels of all time. While sometimes classified as a 'juvenile' novel, it stands up with R.A.H.'s finest.
Juan Rico earns his stripes as a private, then moving up to officer in the Mobile Infantry. The M.I. is an interstellar infantry force trying desperately to prevent humankind from being destroyed by the 'bugs', intelligent spider-like beings who live in hive colonies.
Heinlein uses this framework to discuss the meaning of service and citizenship. The fact that this novel is sometimes criticized for being fascist shows that he has touched a nerve. While going beyond the bare bones of the story, the action scenes are well written and believable from beginning to end.