Joe Haldeman: The Forever War [1999] paperback

Private William Mandella is a reluctant hero, drafted into an elite military unit to fight in a distant interstellar war against an unknowable and unconquerable alien enemy. Mandella will perform his duties and, as he survives, rise through the ranks, but his greatest test will come when he returns to Earth. Because of the effects of relativity, every time he comes home after a few months’ tour of duty, centuries have gone by on Earth, making him and his fellows ever more isolated from the world for whose future they are fighting.