The Forever War

One of the very best science fiction novels of all time. An intelligent and thought provoking allegory for the Vietnam War.

Private William Mandella is a reluctant hero in an interstellar war against an unknowable and unconquerable alien enemy. But his greatest test will be when he returns home. Relativity means that for every few months' tour of duty centuries have passed on Earth, isolating the combatants ever more from the world for whose future they are fighting.