Bhruic, decent AI would very easily be able to tell that I was not out to get them if I only sacked and took their cities in retribution after crushing their invading armies. A decent AI would be able to see that they have gained nothing, and lost territory and men in a war that they started.

I would understand this sort of behavior if they were fighting for survival, but that is not often the case, as the player does not ask for peace in that circumstance. When I am out to conquer a troublesome power (read: egypt), I do not give quarter or offer peace, I annihilate them completely.