I've only played a few games so far, but I've noticed that sometimes the option to Declare War on another Faction is greyed out. What factors into this?

In my current game, I only have one faction left to destroy (Pariden). And I'd like to do so... but the option to Declare War is greyed out, so I can't attack them. Under "Existing Treaties" I only have a Trade Treaty. (And an aside: I can't even cancel the Trade Treaty!). We are "Close," but we don't have an alliance. We don't even have a Non-Aggression Pact; and so it seems logical that I should be able to Declare War!

I've also tried to cast a few strategic spells, hoping they'd piss them off to some extent, but unfortunately I only have a couple that I can cast (Curse City and Frozen Bones), and they don't appear to have any impact on our relations.

(Another aside: Demand Treaty is also listed under Existing Treaties, but I can't do anything with it. I mean, I can't actually demand a tribute..)

Any ideas on any of this, but especially, why can't I Delcare War on these guys?