I think this has something to do with it.
Go to the Races Shipset Folder and Open the (Racename)_AI_Politics.txt and look for the line:
Turns Since Last War Before Friendly Treaty := (Number)
It is different for certain races but usually it's about 20. So in otherwords i think it means 20 turns without any hostile actions between your empire and theirs, i think. Or it might be ANY War

One of the modders or Fyron will no doubt clear that up.
I've never been able to make peace with an Empire I'm at War with. Once or twice they send a treaty suggesting a Trade Alliance which ends hostilities but they've never accepted one from me.
My Advice here is simple. Massive Military Buildup, so if the Big empire does come angrily knocking on your door you'll have a chance. Maybe invest in some intelligence...Or if you're feeling lucky just go invade and take over the smaller empire your at war with, there, No more problem
This Message was supplied by the Deadstar Continuum, For Victory, Honour and Blowing up Agricultural Colonies with No defenses because they were a "threat".
