Quoting NorsemanViking, reply 7
It should not be to easy manipulating another nation to do your bidding. It also need the self interest of it.
This here gentleman, is a very, very good point.
It is hugely important that the AI will think itself first and the diplomacy does not mean that if he likes you like (-100 to 100 scale) +50 he will automatically join you, or even with bribing and promising would join your wars.
AI must be able to think "what's in it for me, in the long run". (and many many other questions).
There is a questions that should be asked always before AI gives answers to diplomatic request's.
Different races would have slightly different questions off course. But in the end, it always should come down to his "survival instincts".
This is why I mentioned the DC (Diplomacy Currency).
Say I am neutral to all races. However I get attacked by X super faction. I ask everyone to join in my defense.
The AI should go through a routine:
Are our cultures alike? No 50
Are they militarily superior/inferior to me and the enemy? They are inferior to the enemy, but Superior to me. 0
Are our governments structured a like? No 25.
How much to I like this faction? Neutral 100
So when I ask them to join my cause they say well we need 175 DC. Of course DC can be converted to iron, silk, research, industrial units, etc.
However I making this example simple, there should be more complicated routines than that.