I read this in the mid 70s from my local library.
The book was set on a militarized world with two factions in an endless war. It may have been a future earth but I think it was set on an alien world. Very vague memory I'm afraid but I remember enjoying it very much and it's atmosphere of endless mayhem has stuck with me. It's among the first sci-fi I've read and I would love to rediscover it. It's not the Star Trek "A Taste of Armageddon" (although I do love that episode). I'm also fairly sure it wasn't The Forever War either.