Unable to declare Liberation War

[grumble]

The Aztecs have conquered Antioch, one of my suzerain friends
Why cannot I declare a war of liberation against them?
(Yes, I have denounced them)

[sercera]

I believe Liberation requires you to have owned or even founded a city that your rival has conquered. So since it was a city-state then you can't use Liberation casus belli (unless you've also conquered that city-state at one point?)

You may be thinking about Protectorate war --- but Protectorate requires you to actually be the suzerain when you declare war. So you have to use a Protectorate casus belli before the enemy conquers the city-state.

Edit: Per the civilopedia, if you or an ally has lost a city to another civilization, you have researched Diplomatic Service, and your diplomatic relation to that other civilization is at denounced and it has been at least 5 turns, then you may declare a war of Liberation against that civ.(It is a war of Reconquest that requires you to have founded that city)