Trade Relationships and War
A bit of realism would not go astray. It is a fact of life that trade relationships reduce the potential for war between nations. If a trade agreement is entered into between two nations, and they are friends, then it should not be possible for war to suddenly be declared, cancelling all agreements instantly. Such relationships should break down gradually. The AI should be configured to reflect this rule.
Realism means no rules in my opinion. Having said that, I think that a heavy positive modifier should keep the AI from declaring war.