Note the city "Aleppo" in the upper right. I am playing the Germans and was at war with the Mongols. I was trying to take a couple of the white cities you see, with no success. A real standoff. They can't reach me, and vice-versa. Then, I get this message that Aleppo revolted, and MY INFLUENCE WAS SUSPECTED. Note the fact my Diplomats can't reach Aleppo, and there is no path of discovered land between that city and the rest of mine. It gave me a free city... 
EDIT: PIC DELETED TO SAVE ATTACHMENT SPACE

EDIT: PIC DELETED TO SAVE ATTACHMENT SPACE