The Forever War by Joe Haldeman. A great example of what science fiction can do when a strong writer gets hold of a killer metaphor. Here's the premise: earth's government conscripts soldiers and deploys them against a dimly understood enemy in a dimly understood intergalactic conflict. When the soldiers return to Earth, many years have passed--due to time dilation, the soldier's "subjective time" elapsed away from Earth is decades less than those who stayed home--and our protagonist feels alien, out of place. To escape Earth he re-enlists, but as he travels more his feeling of alienation from Earth and his younger fellow soldiers only worsens.
Laying in the background of the novel is something of a paean for cultural relativism--for example: Earth goes from predominantly straight and anti-gay, to predominantly straight but gay friendly, to predominantly gay but straight friendly, to predominantly gay and anti-straight, to a place where gay and straight have no meaning. The world gets better, or gets worse, but whether it gets better or gets worse has little to do with culture. It has everything to do with politics, and with the war.
I couldn't help but think of Halo 4 as I read. The novel is something of a repudiation of everything Halo 4 stands for, several decades before Halo even existed. In Halo, intergalactic warriors fight senselessly, and its good--fun, even. In The Forever War, intergalactic warriors fight senselessly, and its terrible, psychologically destructive, and hopelessly stupid. Moreover, the novel continually reveals its violence as senseless, because its senselessness is so overwhelmingly obvious.