In The Forever War by Joe Haldeman, soldiers are sent out on relativistic journeys to fight a war with aliens. Each time they come back, Earth culture has changed, and eventually it becomes unrecognizable to the soldiers, and Earthlings forget or stop caring about the war.
It was an allegory for the U.S. involvement in the war in Vietnam, in which Haldeman fought.
> ... and Earthlings forget or stop caring about the war.
[spoilers]
Not quite. Humans develop (via cloning and other tech) to the point where they can finally communicate with the hive-mind aliens, and realise that the whole 1000-year war has essentially been a mistake from the outset.
> It was an allegory for the U.S. involvement in the war in Vietnam
Yes. The obvious allegory is that to the very few baseline humans who survive the entire war due to time dilation effects the whole thing was a complete waste of time.
A key subplot in the book is that humanity adopts homosexuality such that in the latter stages of the war, the baseline heterosexual veterans are perceived as weird sexual deviants. This seems to me allegory for the difficulty Vietnam vets had in readjusting to civilian life.
The main character is spacially and temporally separated from his love interest/comrade during the war. At the end, he receives a letter saying she's been in cryo sleep aboard a ship experiencing time dilation to wait for his return.
It was an allegory for the U.S. involvement in the war in Vietnam, in which Haldeman fought.