The Forever War (1974) by Joe Haldeman

Wikipedia wrote:The Forever War (1974) is a military science fiction novel by American author Joe Haldeman, telling the contemplative story of soldiers fighting an interstellar war between Man and the Taurans. It won the Nebula Award in 1975, and the Hugo and the Locus awards in 1976. Forever Free (1999) and Forever Peace (1997) are, respectively, direct and thematic sequel novels. The novella A Separate War (1999) is another sequel of sorts, occurring simultaneously to the final portion of The Forever War. Informally, the novels compose The Forever War series; the novel also inspired a comic book and a board game. The Forever War is the first title in the SF Masterworks series.

Wikipedia wrote:William Mandella is a physics student conscripted for an elite task force in the United Nations Exploratory Force being assembled for a war against the Taurans, an alien species discovered when they apparently suddenly attacked human colonists' ships. The UNEF ground troops are sent out for reconnaissance and revenge.

The elite recruits have IQs of 150 and above, are highly educated, healthy and fit. Training is gruelling – first on Earth, in Missouri, and later on a fictional planet called "Charon" beyond Pluto (written before the discovery of the actual planetoid). Several of the recruits are killed during training, due to the extreme environments and the use of live weapons. The new soldiers then depart for action, traveling via interconnected 'collapsars' that allow ships to cover thousands of light-years in a split second. However, traveling to and from the collapsars at near-lightspeed has massive relativistic effects.

Their first encounter with Taurans on a planet orbiting Epsilon Aurigae turns into a post-hypnotically suggested massacre, with the unresisting enemy wiped out. This first expedition, beginning in 1997, lasted only two years from the soldier's perspective, but due to time dilation, upon return to Earth decades have passed.  On the long way home, the soldiers experience future shock first-hand, as the Taurans employ increasingly advanced weaponry against them while they do not have the chance to re-arm.

Mandella, with soldier, lover and companion Marygay Potter, returns to civilian life, only to find humanity drastically changed. He and his fellow soldiers have difficulty fitting into a future society that has evolved almost beyond their comprehension. The veterans learn that to curb overpopulation, which led to worldwide class wars caused by inequitable rationing, homosexuality has become officially encouraged by many of the world's nations. The world has become a very dangerous place due to widespread unemployment and the easy availability of deadly weapons. The changes within society alienate Mandella and the other veterans to the point where many re-enlist to escape, even though they realize the military is a soulless construct. Mandella attempts to get an assignment as an instructor on Luna but is promptly reassigned by standing order to combat command. The inability of the military to treat its soldiers as more than highly complex valuable machines is a theme of the story.