[Pocus]

There is a little chunk of diplomacy in Doms II : you can define in scenarios which nations are friends. This will prevent the AI of striking at a supposed ally, and for the player, until they attack (if they choose too) the given AI, they will have not to fear any attack.

I dont know aside that if something else has been done, perhaps you can end a war against an AI by sending a high enough bribe. Can be coded in a patch I suppose (if they want!)