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.
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