Quote Originally Posted by Barbarian
"Untrustworthy" refers mostly on backstabbing your allies. You should never attack allied faction, if you don't want to become "Untrustworthy", you must first cancel the alliance.
Even cancelling an alliance out of the blue will get a rep hit... If you ever intend to go at war with someone, either do not ally (ever !!) or force them to strike first...

Quote Originally Posted by Zaleukos
Cancelling diplomatic agreements (except for when you have to drop an ally due to a declaration of war).
Same here, being forced to choose between two of your allies going to war will almost always get you a rep hit (from my own experience) which is why you should refrain from allying with parties that are sure to go to war (like Byz and Turks for exemple...)