The trouble with movies is that they fit only a short story. You can make a terrific movie out of a short story, but I have never seen a movie made of a novel that was at the same time good and faithful – most movies made of books fail in both.
The novel Starship Troopers chronicles the evolution of a high-school kid into a mobile infantry officer in a world which had seen modern democracies fail and be replaced by a veterans’ rule (veterans of also noncombat and civilian service, not just military) – Juan Rico gets in just to impress a girl, and maybe to get to vote some day, goes through boot camp, makes combat drops from orbit as a private and then as a NCO, gets opted for officer training and by the end of the book is a competent officer, helping his old drill sergeant who had found the prize. Along the way we get political sermons (now I know where John Ringo got his tendency to have characters lecture on politics!), some interesting characters and just a hint of romance.
The movie paints in broad strokes and primary colors. The first half of it is in fact a fairly decent redesign of the book as a movie, though I did not like at all how much in the face the romance (and two interconnected love triangles!) was played. Johnnie Rico gets in to impress a girl, goes through boot camp, makes some combat drops – And then it transforms into a horror movie (and loses any resemblance with the book). They make a combat drop, Lieutenant Rico drops his mission and goes rescue his girlfriend (the two triangles having been – eliminated by now) and totally misses his old drill sergeant finding the prize. Allegedly, the director never finished reading the book. Well, it shows. In the book, Rico would have been hanged by the neck until dead, dead, dead well before the end of the movie just for striking a superior officer; in the movie, everybody just shrugs it off.
The book is an enjoyable military story, the founding father of a subgenre consisting of lots of newer books; the movie is just silly.
If you have seen the movie, read the book. Don’t bother the other way around.