When one of my allies goes to war, I would like to be able to join the battle to help my ally. However, everytime I move my army into the contested province, the AI thinks I want to go to war with my ally.

I've seen the AI ally against me on many occasions, but I can't seem to turn the tables.

Sorry if this is already posted somewhere, but I couldn't find it if it is.

Thanks in advance
Llywelyn