The Forever War is a science fiction novel about the Vietnam War, as Haldeman cheerfully acknowldges. Published in the 1970s, this work shows its age. The characterization is clumsy and some innovations have been ripped off so many times they now seem cliched. But it's a classic in the field, and a must for anyone who wants to be well-read in science fiction.
Mandella has been drafted (just like the author was in 1967) into a futuristic army that only wants the best of the best. Then it puts them through hell. Mandella and his cohort are involed in a war whose cause they don't understand, fighting an incomprehensible enemy. The downside? At relativistic speeds, two years on the march in space means over twenty years has passed before he gets back to Earth. He comes back to a totally different world. Nobody understand him! Everything has changed! Etc!
Like I said, it shoes its age. This is not groundbreaking social commentary - any more. In 1975 it was another story.
Mostly historical interest.