The story is set on a planet where time passes much more slowly at the pole. A soldier is near one of the poles and a major offensive seems to be building up with his side firing into some kind of grey curtain, a horizon caused by the extreme time "gradient". The other side is firing back. He is fetched from his post as his discharge has come up and travels to the equator where he starts a new life. 20 years pass when 2 officers approach him, they had left to get him back only a few minutes after he has left and he returns to his post some 10 or so minutes after he had left. He realizes that the weapon fire they are getting from across the void is just there own stuff coming back. I have always taken it as an analogy for lots of tit for tat wars, but cannot find the story.