I'm not so sure about this. Currently, wars happen due to a clash of ethics. Goblins and kobolds have ethics that cause them to be at constant war with everyone else (kidnapping and stealing). In fact, you can make the goblins friendly by either deleting that tag or adding it to your dwarfs, you even get goblin caravans bringing troll fur socks in the winter (its actually already in the game, goblins are just always at war). Elves declare war on the dorfs for cutting down trees, dorfs declare war on the elves for being dirty hippies (oh, and the whole cannibalism thing- dorfs respect remains of sentients). Humans find slavery and torture acceptable, and don't mind if you kill a sapient for a good reason. They also find collecting of trophies to be acceptable. They do find EATING people to be abhorrent (like dorfs) so they often war with the elves too. However, that trophy taking, killing sapients sometimes ok, and slavery stuff sometimes cause them to war with the dorfs.
Again, all of the current wars are based on a conflict of ethics. What happens when you want to declare war on a human civilization that doesn't collect trophies? Or could you declare war on another dwarven civlization? I dion't think your Urists would need a reason to butcher the elves, but they do sometimes bring gifts of giant tigers. My point is this, just declaring war for shiggles doesn't fit into the game wanting you to declare war on infidels (extreme conflict of ethics) or acts of aggressiion (butchering caravans).