[Shadowstar]

Well, not every AI empire needs to declare war right away. It's just one variation of the agressive persona. You'd expect a race with lots of early combat advantages to do it. They'd start building warships on the first encounter with another race and strike when they were more militarily powerful than that empire. Later wars would only break out after a considerable amount of buildup, and in those cases then you could have global-themed wars like genocides and such, that span the entire map, rather than one neighbor trying to take out the other.