I've noticed that the AI likes to throw all of its armies at you the moment you attack; I've used this to my advantage several times. On turn one after declaring war, I'll enter at the far end of an enemies territory with a largely defensive force; this draws the AI's armies away from where I really want to attack. On turn 3 (after giving time for the AI army movement) I'll then attack with my offensive force at the opposite end of their territory. By the time they've figured out what is going on, I've taken several of their cities, and they've wasted a bunch of their armies trying to destroy my defensive force.