Trying to remember a short story (most likely from the sixties). It starts with a soldier at the front line fighting an unknown enemy on the other side of the front line.
He then gets withdrawn and you realise that time is passing slower the further he moves from the front. He then spends years in peace away from the front, before being sent back to the front where only a short time (hours or days perhaps) have passed.
I seem to remember their was an inference that they were actually fighting against themselves, with their attacks coming back at themselves, which was perhaps the entire cause of the war, and an interesting critique on war. I can't remember if this all this was stated explicitly, or just left for the reader to infer.