Quote Originally Posted by Geezer57
It doesn't happen all that often, but I've seen the AI do pretty much what you described on several occasions. They often stay to see how you deploy, but if there seems to be no good way to attack you advantageously, they'll leave the field of battle.
In your case, there must have been a sea link to their last province, which allowed a successful withdrawal.
I see that all the time. You notice it more in woods an on hilly terrain. The AI doesn't get a good look at your entire army, until you're getting close to them so they manouver, deploy, redeploy, and then when they get a clear line of sight on your entire force, they withdraw.