Quote Originally Posted by Asai Nagamasa View Post
It isn't possible to coordinate with the AI, but the AI may join you if you invade an enemy province. In such cases it's wise to see what kind of force the ally has brought along. If they have deployed a larger force then yours - they will win the province. If it's a smaller force you should win it - but you need to ensure that the ally does their share of the fighting. It's not good when your units do most of the fighting and dying and the AI ally wins the province.
You can use it for your own good. You can attack with smaller force than your ally and let him do most of the fighting. If your forces after the battle will be greater that your ally, you will get the province. I like to keep my enemies and allies low in numbers because even your ally will become your enemy sooner or later.

Gilrandir If you and your ally are in war with the other faction and your ally has a possibility of sending an army (he can attack the same province), he will very likely do this. Of course it depends of your king's influence, happiness in the province when the army stays, etc. If your ally is neutral to your enemy don't count on him.