Originally Posted by Cheet4h
I'm currently trying to make two races peaceful to each other, as they are having a war while they share a common alliance partner. If I choose "Make peace with ..." and select the race, they state, that they will gladly make peace and confirm my offer. But even if I do this with both of them, they won't go to peace, they are still at war ...
Are either of them Ruthless? If so, there's nothing you can do to get them to back off. One of them has to go.
If not, try to get them to make peace while the game is still paused, exit the game, and then resume. From experience, there are sometimes residual "effects" in play that will cause races to declare war. Resuming the game either clears the effects out or clears out the "owner" tag so that they're not associated with the race that caused them.