Make truce's more permenant.

When your alliance presently makes a truce with a warring alliance, you cannot attack them. This is good! But, one easy exploit is to make an unreasonable demand of someone in the alliance. When you "sue for peace", if it's agreed to then it should be more permenant than this to give the losing side some kind of recompense for what they have to give up in the deal (such as a system), and give them a chance to build back up.

One easy way to do this would be to enforce truce's for 20 turns, disabling diplomatic aggression, and automatically imposing closed borders. (the upside for the aggressors would be whatever demands are conceded, plus the ability to focus their forces elsewhere on the map).