Despite their open aggressiveness, i've found that AI factions are woeful at taking advantage of the player's weakness.

In my milanese campaign, at turn one i moved everyone out of my cities to destroy the papacy, and none of the neighbouring factions moved in to take the open cities.

Foward 20 turns, and i'm the undisputed military leader, so i'm betrayed on all fronts.

Huh? ;)