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Ok. So I read books as a kid, this may have been mid to late 1980s. Definitely before 1993. These details I remember, they might be from the same book or from several:
Slower than light space travel with severe time dilation. There were boardings of space ships, but the fighting odds could be very uneven because a foe could be from your future or your past, depending on when you started your space journey, because of relativistic time dilation. No faster than light travel, no time travel either. Just regular physics.
To fight better in melee, soldiers would have (more or less willingly, I think they knew beforehand) false memories or notions implanted of the enemy as horrible human beings so soldiers could kill them more easily without remorse.
Towards the end of the story, veterans of the war are described as not quite feeling at home after the war. One planet (home?) or country is described as inhabited entirely of clones, Adams and Eves, I think homosexual, or that sexuality could be switched deliberately. I think the veterans find at the end a refuge somewhere more like the home they are used to. The reason so much has changed is of course the time dilation. They spent maybe decades in the war, while on the planets centuries or millenia passed.
I think one moral (there may be many more I missed as a child) of the story was the futility of war. If I recall correctly, the war was resolved in a way that made the efforts of the veterans seem for nothing.