A ninja village is equivalent to country's military in the naruto world and their income's main source are people and feudal lords who contract ninjas for all kinds of work like assassinations,protection detail and other stuff. Say two opposing parties hire ninjas from two different villages to do opposing work like like say Lighnting country ninjas have to protect some person and Konoha's ninjas have to kill him( and i'm guessing this happens frequently,ninjas couldn't be catching cats all the time,it doesn't pay well as assassinations) wouldn't that lead to conflict? And someone is going to end up dead? I can't see how ninja villages can be at peace with each other unless they're cool with their guys getting killed in the line of duty by folks from other ninja villages? Plus,what will ninjas do for pay, in peace time,make baby bottles?