Diplomacy and keeping relations up

[dopp]

I think it was if the AI faction in question was at peace with everyone, it was forced to declare war on the human player.

If you are at war with someone the Pope will force a truce on both sides. Usually this means the attacking armies will start moving off your territory, but any sieges and blockades already in progress are not broken for some reason (maybe an oversight, since sieges/blockades are broken when you sign a ceasefire). The game is smart enough to recognize defensive actions. You can attack any enemy army besieging your towns or fleet blockading your ports. You can also attack any enemy army you find wandering around on your own territory. You can even wander around enemy lands as long as you don't pick fights with anyone. The Pope is actually quite easy to please.

The game is also smart enough to recognize when you take advantage of (or provoke) an enemy attacking you to defeat his armies and invade his lands. You will get reputation hits for doing anything other than self-defense.