In The Forever War by Joe Haldeman, you are given the story of a soldier fighting a far off enemy for reasons he does not understand. But in fighting that enemy, he is rapidly alienated from his own species by the progress of time and his lack of aging due to relativity. There are many direct associations that can be made between this and the Vietnam War (for reasons of the Author's involvement) and many have made comparisons between this and Starship Troopers by R.A.H. where the focus of the comparison has been on Starship being more optimistic while in Forever War there is a general sense of dread and then just resolve.
This book was excellent. This is the first time, in my relatively short time of reading Science Fiction, that a book gave me very real sense of dread about the future. On the main characters first trip back to Earth, he is faced with a very different world that has been changed in the 30 years he was gone by food shortages and rising class violence and lawlessness because of how food is handled. Just reading the causes and effects that Haldeman lays out still gives me chills at how very plausible it is.
Of course this trip back to Earth really sets forth the desire for the main character to re-enlist just to leave behind an Earth he does not know. And for then on out, you are given glimpses into the way that the worlds change as our hero finds himself out on longer and longer missions, where he just loses touch with what humanity has become.
Overall, this is definitely a book I can recommend. Don't be put off by the timeline as it is still an amazing story and setup. Also I recommend to make sure you get the latest author definitive edition as that covers alot more of the negative future aspects.