[Strategia_In_Ultima]

What I heard is that the AI won't accept peace treaties within 1 or 2 years after you start a war, so if you two do not experience any actual combat action in a year or two, they might accept a treaty. Also, if they don't, you can offer a trade where you say you WANT a Non-Agression (or perhaps even Trade) Treaty and you are prepared to GIVE, say, resources, starmaps, tech they already have, ownership of systems etc. (What I usually do is offer them "non-existent" things, like starmaps I know they posess, tech I've seen them use, badly damaged outdated ships, ownership of nebula/black hole systems etc.)