I've been playing as Dunland, trying to get Tharbad to submit to protectorate status. So far no luck.
I started a war with them early on and quickly got them down to 1 settlement, Lond Daer. The garrison consisted only of 3 FMs, and there are no other military (land) forces around as far as I can tell. Tharbad still has a small fleet, consisting of 2 ships, at least 1 Emissary, and at least 1 Scout (Spy).
I have besieged their settlement for a few decades now. I have a full stack besieging Lond Daer, and another half-stack standing in the port to blockade it (as I have no navy). I have made perhaps a half-dozen demands for submission, offering as much as 94,000 mirian, and occasionally offering back settlements. Tharbad *does* seem interested in a ceasefire (but they want me to pay over 10k for it).
Tharbad had no enemies (except me) and no allies until recently, when they allied with Adunabar. Adunabar isn't *that* strong, though, I don't think, so this misplaced sense of pride on Tharbad's part doesn't seem like it would be coming from their alliance.
So I wonder if their navy is making them feel stronger than they really are. My next goal is to get some boats together and take care of those 2 Tharbad ships, and see how that works. My treasury is close to 100k, so maybe I'll just need to offer more cash. But I wonder if those ships make the AI feel as though it has more options than it really does.
Edit: Failed to get Tharbad to submit. They were down to just one elderly faction leader, and zero other FMs or military units. I built some boats and destroyed their 2 ships. Offered them over 200,000. No dice. The FL died and faction was eliminated.
Very disappointing, that. I'm not sure what's the reasoning. I've encountered stubborn factions who eventually submitted (see Dunland and Rhun, above); apart from this Tharbad experience, though, the only faction I've consistently had trouble with getting as a protectorate is Khand (trying as both Harad and Rhun, as detailed in above posts).
Maybe some factions are simply more stubborn than others. Maybe it's other considerations - Tharbad was only at war with me, and perhaps our relations had simply deteriorated to the point where they couldn't accept. As a prelude to our war, I demanded some money; I refused, and attacked, so maybe the intimidation factor really hurt our relations. We also never had a ceasefire during our war, though they wanted one (along with territory back, which I refused). So maybe I should have taken a ceasefire and just immediately continued the war? That has worked in the past for me (see my experience as Dale).
My guess is that the AI values ceasefires pretty highly (not that it *respects* them for long, but it certainly seems to think it's worth a lot of cash and, often, settlements); maybe the boost in relations for getting a ceasefire is enough to counter the presumed damage to relations by attacking the AI after a ceasefire has been signed...