I think, going into direction of "even more smart" is pointless...
Well, your AI is very smart within the parameters it evaluates...
But is the end result really that smart?
How rational is it really to grind billions into a never-ending war that gain nothing?
I'd say... not really.
It sounds a bit like your "perfect AI" need to read "The Art of War" to start with
My point is just that your AI need to take more parameters into consideration, and that would mean being "smarter", not trying to make it less smart. Lorenzo again gives good examples of what these parameters could be.
Other parameters could be that which we call "emotions". A never-ending war will for example likely run your troops morale into the ground, which makes your war even more costly and pointless (unless everyone is robots, then I guess a never-ending war is plausible, but still sounds more like a programmer bug on the part of the creators of said robot civilization). Technically, this is just a float somewhere that get reduced every turn the war continues and modify the "importance" of this planet, but it is more fun and useful to think of it as "morale"
Another way to say it, I guess, is: The game should be balanced such that stupid and boring strategies simply isn't perceived as advantageous. Then the AI won't pick them.