My 7 year old son and I have pitted the Elves against the Dwarfs -
The Elves control the sky - they have airships, gliders in their cloths that enable gliding from airship to airship, and bridges between the airships.
The Dwarfs control the underground - living in great underground caves, tunneling from place to place via giant tank-like vehicles with drills attached to their fronts and ends.
Between the two races, earth and sky, floats the ominous Everboom - a perpetual cover of stormy clouds, where lightning cracks every few seconds, everywhere (which is a good reason to keep to the underground and very high in the sky).
The two races are at war on the ground (still figuring out the details of safely moving through the Everboom), so there should be battles and victories galore, and so on and so forth, and glory to the young story teller. But the teller would have none of that -
My son wishes for the two races to be at war but to never be able to complete a battle. In other words - military engagements without a defining outcome. He simply doesn't wish for either of the races to win in any battle, saying that the soldiers shouldn't want to fight - though obviously, he wants them to have do go to battle. The reason for this is a mix of a kid's excitement over cool fighters with cool fighting techniques + cool shiny weapons, and a pacifistic gentle soul.
So I was telling him about evil generals that force their unwilling soldiers to go to war, to which he justifiably replied that the soldiers won't have cool fighting techniques + cool big weapons, if they don't really have their heart in the fight. I started suggesting some honor system that would make it viable to have a warrior caste where the guys want to fight each other for honor without killing or something like that, but the boy is too smart and realized that in such engagements, there would be winners and losers and deviations from the honor code, and these would lead to killings and taking-overs of desired resources and/or land.
We need armies that wish for war, will ferociously have it out at each other, and when their opponent is vulnerable for the final blow, will say: "See you next time" and go home. Kind of like in kids' tv shows.
Sigh - What would be a good reason for a couple of races to want to have serious battles, without wishing to defeat, or take anything from, their opponents? I would love a sociology/psychology answer, but will prefer one that includes the world's unique physical aspects (the sky/underground factors, the Everboom storm and methods of safe travel through it).
There's a bit of magic to be had in the kid's stories, so a very minimal amount of magic is allowed for the answer, but no "A-wizard-cast-a-mystery-spell-over-everyone's-mind" - We're 7 years old, you can't hand wave our troubles away.
EDIT: Few casualties are allowed in the fights, but everyone will be childishly shocked and sad when they happen.