Go from guarded to friendly

I've been playing a world for quite a while now, and I'm trying to be friends with Greece again. For a long time we were friendly, and then when I accidentally nuked China (I forgot about the blast radius and the dialogue of "does this mean war???" didn't show up so I assumed nothing would happen) the other two most powerful militaries in the game (Carthage and Greece) became Afraid of me. I was fine with that because I just wanted trade deals and to be able to conquer other civs with them, when Carthage backstabbed me I was fine because in the military rating I had five times the number of points, but after Greece and I conquered Venice and were in the middle of conquering the Iroquois, they became guarded instead of afraid. The negatives now are coveted lands, different ideologies (This happened after guarded started), warmonger, and failed proposal. But, the positives are fought a common enemy, denounced a common enemy, Traded together (highest point because out of frustration I gave them a city they tried conquering but I bought and 150 GPT, I had 700 so I was fine). but nothing will change. I want to become friends with them because Carthage has been sending a lot of troops around my capital and probably will declare war on me soon enough, and our defensive pact ends on 3 turns. Afraid would be good too.

Well, I'm not much of a player, but I think one of the key things to remember is that the AI civs are also trying to win the game -- if you become a serious competitor or obstacle to that aim, they will try and kill you, and nothing will change that. It's just a consequence of being perceived as a threat. If you like, it's a natural consequence of doing well. Very Hobbesian!