The Forever War?

That's because I read about The Forever War on Wikipedia a few years ago and forgot the title, then when I saw this movie was released I thought that erroneously remembered the book title being "The Tomorrow War."

I don't think it is at all. I just read the synopsis for The Forever War on Wikipedia. It sounds good .. which is one reason this is not a rip-off ... The Tomorrow War stunk. Here is a short excerpt from the description of The Forever War ...
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... Their first encounter with Taurans, on a planet orbiting Epsilon Aurigae, triggers their post-hypnotic training, which causes them to massacre the Taurans despite their lack of resistance. This first expedition, beginning in 1997, lasts only two years from the soldiers' point of view, but due to time dilation, they return to Earth in 2024.[4] During the expedition's second battle, the soldiers experience future shock first-hand, as the Taurans have much more advanced weaponry. Mandella, with fellow soldier and lover Marygay Potter, returns to civilian life, only to find humanity drastically changed. He and the other discharged soldiers have difficulty fitting into a society that has altered almost beyond their comprehension. The veterans learn that, to curb overpopulation, which led to class wars around the world 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 mass unemployment and the easy availability of weapons. Alienated, Mandella and many other veterans re-enlist, despite the extremely high casualty rate and their recognition that the military is a soulless construct. Mandella and Potter receive promised postings as instructors on Luna, but upon arrival are immediately reassigned to a combat command.

Almost entirely through luck, Mandella survives four years of military service, while several centuries elapse in real time. He soon becomes the objectively oldest surviving soldier in the war, attaining high rank through seniority rather than ambition. He and Potter (who has remained his last link with the Earth of his youth) are eventually given different assignments, meaning that even if they both survive the war they will likely never meet again due to time dilation. After briefly contemplating suicide, Mandella assumes the post of commanding officer of a "strike force", commanding soldiers who speak a language largely unrecognizable to him, whose ethnicity is now nearly uniform ('vaguely Polynesian' in appearance) and who are exclusively homosexual. He is disliked by his soldiers because they have to learn 21st century English to communicate with him and other senior staff and because he is heterosexual.