I had an even more gaulling (sic) example:

Playing as the Seleucids I was allied to Armenia and Pontus. Armenia attacked Pontus, I was asked which side I wanted to support. I said "Let's stay with the noble wronged Pontus against the deceitful Armenians."

Next thing I knew, Pontus had marched a stack into my territory and attacked me!

Just one more Middle East peace plan in ruins...