In my current game as Poland, me and Spain were all alone on a rather big island, with ocean all around us. No other civs. At all. And no Kamehameha in game, either (I make a point of turning that DLC off >.>). So anyway, at 2000 BC or so, I have a few archers setting up around Madrid as well as two Spearmen. This force (4 archers, 2 spears), Isabella sees as highly threatening despite none actually AT her border and tells me to pull back. I kindly refuse, and attack her five turns later. Taking out both Madrid and Barcelona.
Now, not only was our little continent ocean-blocked, Madrid happened to be surrounded by Ice, and Barcelona was landlocked. And with her entirely wiped out of the game, nobody would know, right?
Fast forward to the Rennasaince, where I send out a few Caravels to find some potentional trade partners (thank god for city states...) and I run into a continent with Shaka, William, Dido, Maria I, Maria T and Boudicca.
Each of them have the negative diplo modifier "You have broken a promise you made to another Civ".
HOW THE HELL DO THEY KNOW?! I mean, BNW punishes warmongers enough as it is, but even punishing warmongers when there's no way for the other civs to even KNOW what happened? Sure, they'll know I wiped Spain off the map by checking my city list, but...this should really be fixed. Come on Firaxis.