Quote Originally Posted by JoeyBritt
My guess is that the better your relation with an allied faction, the least likely they are to randomly attack you
I agree. I have been an ally to the HRE for over 50 turns now and we share the same border. That particular city of mine has minimal defenses, yet, they have never touched it or even approached it.

A small gift of 5000 florins makes your ally really happy.