The Forgotten War by Joe Haldman

The forever war was written by joe haldman, the author of hugo award winning 19 74 military science fiction novel. The book deals with futuristic warriors cast into a conflict in a distant star system against an unknown alien enemy. There's discussion of how time dilation impacts war. So like every time you travel to meet your opponent, they get a leg up on you because they have more time to prepare than you do.