Confused why I can't request a joint war

[Jman5]

China was my friend and I tried to get him into a joint war against Norway, but there was no option in the diplomacy screen. He must have gotten the tech for it because he had Autocracy, and he had a sour relationship with Norway too. Later, we became allies and he still wouldn't help out. He never denounced me.

Can someone explain to me how joint wars work? What has to happen to make them available?

 

[White Out]

ya I notice that the joint war used to be easier to ask for, now, i notice it only very rarely as an option

 

[UWHabs]

For some strange reason, I've found out that joint war doesn't work against someone who you're already at war with. If anyone knows how to essentially say, "hey my friend, would you like to join me in my current war against X", please let me know. That would be a much more useful option than what the current one does.