Deau
Emperor
I've tried both of these methods almost non stop with practically every civ. It's always that theres no way to make it happen. Oh well, I guess I'll just have to keep trying.
It's probably because you were asking a military weak civ to attack a strong one. If the AI you want to see being attacked has highest military strength in the demographics table, it's almost impossible to convince other civs to attack him. What you can do, however, if you are not at war with that given civ, is to bribe that civ to DoW your friend.
Same result, often for a cheaper price. I've had deity games where I would bribe alexander into DoWing 6 AIs the same turn because he was top military demographic