The A.I. has been programmed to target the player. This makes sense in itself, as the player is the greatest danger to the A.I., but the way it's implemented is stupid. The A.I. will ignore its own security in order to get you, and will make peace with factions that are at war with you.

However, I found that I could put an end to most opportunistic attacks by paying neighbouring factions a small, regular tribute (200 mnai works). Even if they do attack, they are more willing to sign peace afterwards. I also get the impression that extensive use of spies and assassins annoys other factions, even if they don't get caught. I haven't tested this yet, though.