The Forever War

Private William Mandella is a reluctant hero in an interstellar war against an unknowable and unconquerable alien enemy, but his greatest test will come 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.

"The Forever War", published in 1974, won both the Hugo and Nebula Awards for Best Novel and cemented Joe Haldeman's reputation as one of SF's major writers.