In this current game I'm playing, Norway was the only AI that wasn't unfriendly to me.
HE contacted me asking if I could start a joint war against Scythia. I refused (I didn't have good enough units then). Not 5 turns later, I ask for a joint war with Norway against Scythia. They agree and we go to war.
The very next turn he denounces me because I am a warmonger
The funny thing is that after the war is over (which of course he fully participated in yet did nothing intelligent), I have +5 for "We fought a joint war together" but -26 due to "[my] warmongering".
It's like there were 5 separate people working on the +/- relationship modifiers and never talked to each other throughout the development process. No balance whatsoever.