Joe Haldeman's "The Forever War" exploes this quite well. With relativity taken into account, a 18 month tour of duty takes years for those not whizzing around space at 99% of c. What was cutting edge when they left is crude when they return, most noticably weaponary, tactics used by the enemy, and the nature of human society. Well worth a read.
FTFY. And yeah, I loved the idea that there was NEVER technological parity between Earth and the Taurans. Defender almost automatically had technological superiority, because they've had time to develop and deploy new weapons and tactics while the attacker has spent all that time in cryosleep and travelling. The only time they're even roughly paritible is
the last skirmish, where technology and countertechnology has advanced to the point where they're literally fighting with spears and darts inside of a zero-energy bubble thing.
It follows that in a war like that (as in WWI) it would be bloody, long and little ground would change hands because the defender always has a significant advantage.
The comic book series
Albedo also had pretty realistic space combat. Basically, both ships manuever (within the G-tolerance of their crew annd their fuel budget) for optimal vector, then shoot a metric shit-ton of metal chunks at each other at relativistic speeds. Much of their cloud of drones and slugs will hit and annhilate each other, but some will get through. You basically shoot and pray that your cloud of ordnance was bigger than theirs, then both sides strap in and wait a few hours for impact. I think Forever War had some of that as well (I seem to remember one of the missions is basically FUBAR'd from the start when they get hit by relativistic shrapnel and lose a number of their ground troops before they even get to target).