The Forever War is a 1975 Hugo Award winning science fiction novel by Joe Haldeman, the first book of the Forever War series.
Warning: The following paragraphs contain spoilers
It tells the story of William Mandella, a university student conscripted for an elite UN task force being assembled for a war against the Taurans, an alien species that was discovered when they suddenly attacked some Terran colony ships. Because of the unknown nature of the threat, many cadets are recruited for their unique knowledge and talents, including telepathy and luck. After a grueling training regimen that results in a number of casualties, the recruits finally ship out.
Due to the physics of deep space travel, the war lasts for more than a thousand years from Earth's perspective. Over the course of the war, the soldiers (who because of the relativistic effects of faster-than-light travel suffer severe time dilation) experience technological future shock firsthand in combat as the Taurans employ superior weaponry against them.
Eventually the soldiers complete their tour of duty and are shipped home, where they have difficulty fitting into a future society that has evolved almost beyond their comprehension. Among other things, the veterans learn that to prevent overpopulation, homosexuality has been officially encouraged by the world government and later, made the societal norm. The changes within society alienate the veterans to the point where many re-enlist.
The war ends after mankind evolves into a clone-based species calling itself Man and is able to communicate with the Taurans (who turn out to also be a clone-based species).
The novel is widely perceived to be about the author's military experiences during the Vietnam War, although set in a science-fiction context.
