nafod's right that the AI's declarations of war, and subsequent pursuit of same, is sometimes non-sense. It should be able to bring war to end (victory or defeat), or else internally decide on peace treaty after 10 years or so. but perhaps this is just symptom of lack of invasions- they'd likely cause wars to reach resolution.

I don't think AI needs to be harder, just smarter. (but then again, I'm clearly not one of the most aggressive, or effective, players...)