I posted (in the standard BtS forum) some time ago about this happening to me before, and it has happened again under very similar circumstances. The situation is as follows: I'm relatively friendly with another civilization, and the very instant that they create a colony/vassal civ, they issue a declaration of war against me. They don't actually launch an invasion, just a declaration of war (if I reload a previous save, the war still comes as soon as they create the colony). In the first case, it happened with the Native Americans; this time it happened with the Greeks (under Pericles). My relations level with the Greeks was Pleased (and I think the same with the Native Americans last time), and there's no other reason that I can find for the war declaration. I wasn't in any other wars, we had an open borders agreement, we were trading partners (and had been for a very long time), we were far away from each other, etc... Like I said, there's no invasion force that I can see, just a random declaration of war at the same time they form a colonial civilization for a vassal. Edit: Just like when this happened to me the first time, I do not get any popup diplomatic window with the other civ's leader telling me that they're declaring war. Also, I reloaded the game from the turn before his colony creation + war declaration, and asked if he could spare his map info for a good friend. Pericles agreed, and this got us the "10 turn peace treaty" agreement. I end the turn, and lo-and-behold, he declares war on me at the same time he creates his colony (this time it was Mayan, instead of German; everything else remained the same). This is really annoying, since I do not want a war with the Greeks (not that they actually have an invasion force assembled, mind you).