I am trying to track down a sci fi short story from a magazine.  Been bugging me for years.

Not sure what magazine - journal size 7-8" x 10-11" - approx 70 pages when I picture holding it.  I got the magazine some time during 1977-1979, though the cover was missing.
The father of the person I got it from was a collector of magazine cover artwork.  So older than 1979 but not sure how old.

**Plot:**

A man in his 20's wakes up to find he is in the middle of a World War I-type battle.
However it is not WWI.  He is on a 'war planet' where disputes are settled. 
Rather than all out war, the two sides select war conditions and fight it out on this planet to resolve the dispute.
The rules selected for this battle was WWI conditions.

- Not single combat (like Arena).
- Many 'soldiers' were there for both sides.
- Not sure if disputing sides choose the planet (I think this was the case) or if another advanced species forces it (like Arena).
- I think the person didn't volunteer to be part of the fight and may not have even been from one of the two races in the dispute.
- The person didn't really know what he was doing or clearly understand fully why he was there.
- I remember a grenade being thrown in the trench and shoved into a grenade pit as a key event.