I'm trying for a diplomatic victory in my current game. All the AI opponents seem to love me, which is hard to maintain. But I'm starting to see that even when they're my friends the AI are dicks.
 
Playing as Greece, I've stayed friends with Japan for most of the game. It's at a point where they give me great trading deals and would go to war for me if I asked. The problem is that they keep fucking with my allied city states. I've lost 3 allies to them which is a big loss of investment. No matter how many troops I supply to the city states, they can't stop the Japanese juggernaut.
 
I rely heavily on city states to feed my growing empire and Nobunaga's fucking up my strategy while calling me 'friend'. There's got to be a way to stop him without going to war. The game keeps graying out the option to make peace. The rollover always says something about "permanent war" being the reason. I'd ask for peace through diplomacy, but the game won't allow it. Any ideas?

You can't ask him to make peace because he has attacked so many city-states they all turned on him. Try to "pledge to protect" the city state, that might deter Nobunaga from going to war with them.

I constantly pledge to protect, but he keeps boldly walking into their lands with trebuchets and samurai. Hasn't done much to slow him down.
 
To his credit, he will always acknowledge what he's doing to me and apologize for stepping on my toes. But his attitude is totally "Sorry dude, I've got no other choice". Total bull~shit. I'm playing on a large map with plenty of room. He's got valuable resources right next to him that are unclaimed.  Guy just wants to make war for the fun of it.
 
I'm probably going to have to take him out. He's begging for it.
 
At least city states will love me for it.