The Kingdom of Cornwall was one of the realms that challenged Arthur's ascension to the throne. And it is a member of the Supreme Collegium (and thus presumably is expected to be included as part of the realm whenever a High King is chosen.) So how did Cornwall maintain its independence from Arthur until 544? What was Arthur's reason for not finishing the job after Terrabil and forcing Cornwall to submit? It's hard to see how Arthur can credibly claim that the rebellion against him is over when such an important part of the realm refuses to accept his authority. (And if nothing else you would think that Arthur would at least have insisted on Cornwall relinquishing the territories Idres conquered from Logres during the Anarchy, but Arthur doesn't even seem to have done that much.)

Here is a suggestion...

In 512, Sir Mark, "one of the royal chieftains of Cornwall", is met right outside Arthur's Court. This to me implies that Mark might have been more friendly with Arthur at the start, or at least keeping a foot in both camps. This would explain why after 513, with Idres presumably dead at the Battle of Terrabil and Cornwall erupting in a civil war of sorts (Brastias' comment in 514), Arthur doesn't do anything; he already has his 'man', Prince Mark, fighting to establish order. Mark is crowned King in 515.

Griflet's comment in 524 hints that all is not sweetness and light, though, hinting that they might invade Cornwall to help King Anguish against King Mark.

And of course, there are the events in 543 and 544 that lead to king Mark swearing homage to King Arthur.

So here is how I'd play it:
510-3 Mark as a lukewarm Arthur supporter.
514-5 Arthur wishes to preserve his army and doesn't pursue the Cornish option: he has his work cut out for him pacifying the Northern kingdoms. Note that he doesn't annex Malahaut (completely) either. King Mark ascends on the throne. Arthur starts prodding Mark about those counties that were part of Logres during Uther's time. Mark comes up with excuses. Also, Arthur is getting married and is building Camelot.
516-8 Saxon Wars, Arthur is busy.
519 Arthur busy fighting the Picts.
520 Anglian revolt.
521-4 Perhaps Arthur prefers peace rather than push Cornwall. After all, Logres has seen almost a decade of constant war already. Time for a break. Towards 524, Cornwall starts to loom in Arthur's thinking again, but the delegation of the Roman Emperor insults him.
525-8 Roman War.
529 War against Galehaut.
530 Irish War.
531+ The Cornish Question is getting to become so old as to become an 'established fact'. Live and let live, at this point, until blind chance delivers King Mark to Arthur's power in 544.

Other options:
1. Those counties up to Somerset were originally part of Cornwall, until Gorlois expanded there. So Mark can claim that justice is on his side, that Idres was just returning the border whence it was.
2. Mark does hand those counties over / Arthur demands/reconquers them in 514. Adjust Cornwall's borders accordingly. Problem solved.

So how did Cornwall maintain its independence from Arthur until 544? What was Arthur's reason for not finishing the job after Terrabil and forcing Cornwall to submit? It's hard to see how Arthur can credibly claim that the rebellion against him is over when such an important part of the realm refuses to accept his authority.

I think you are mistaking the Duchy of Cornwall with the Kingdom of Cornwall
Everyone seems to do this
King Uther does take all of the Duchy back into his own realm
Then in Anarchy the King of Cornwall takes a big chunk of his former realm (called Kingdom of Cornovii then) back: Tintagel, Ascalon, and Jagent
The Kingdom of Cornwall is actually the Kingdom of Cornwall and Brittainy
It is pretty powerful, and no doubt King Arthur would rather not attack it directly, for political reasons

I think Cornwall lucked out partially because it's in the opposite direction from the rest of the kings fighting Arthur.

Arthur doesn't immediately attack go after Ireland either, even though they were on Lot's side as well. He's not vengeful, and of course - even in victory he lost a lot of knights.

On the maps it does kind of look like at least Jagent reverts back to Logres. The earl of Jagent is listed as one of the vassals that swears to Arthur in 510.

It seems like it's also worth noting that while Arthur takes out the major political powers of the north, the conquered territory seems to be rife with petty warlords and robber barons. (The better to trouble the noble questing knights of the realm, of course)

Also Saxons. I'm betting if he was contemplating going down south to take back the Duchy of Cornwall from the kingdom of Cornwall, the Saxon invasion in 516 abolishes that idea. (And since the Cornish have allied with the Saxons in past conflicts, he might have quietly cut a deal to keep them out of it)

tl;dr

Politics.


What was Arthur's reason for not finishing the job after Terrabil and forcing Cornwall to submit?

The Battle of Terrabil during the rebellion takes place in Cameliard, this is a different Terrabil than the Terrabil in Cornwall that figures into events during the Uther period. There are many Terrabil places in the world it seems.

Arthur doesn't immediately attack go after Ireland either, even though they were on Lot's side as well. He's not vengeful, and of course - even in victory he lost a lot of knights.

True, but the difference is that none of the Irish kingdoms were legates of Britain whereas the King of Cornwall was one of the legates (per the list of the 28 legates who make up the Supreme Collegium in the GPC.) Thus Arthur can make peace with the Irish and let them maintain their independence without lessening his authority as High King of Britain (since Ireland isn't part of his realm anyway), but doing the same with the Kingdom of Cornwall weakens his authority as High King (since the Kingdom of Cornwall is properly part of his realm as High King elected by the Supreme Collegium.)


Also Saxons. I'm betting if he was contemplating going down south to take back the Duchy of Cornwall from the kingdom of Cornwall, the Saxon invasion in 516 abolishes that idea. (And since the Cornish have allied with the Saxons in past conflicts, he might have quietly cut a deal to keep them out of it)

That's possible. Arthur might also want Mark as a potential ally against King Claudas since Mark has substantial holdings in Brittany.


514-5 Arthur wishes to preserve his army and doesn't pursue the Cornish option: he has his work cut out for him pacifying the Northern kingdoms. Note that he doesn't annex Malahaut (completely) either.

But he did make Malahaut swear fealty to him. It's not requiring the same from Cornwall that I don't understand. I could understand Arthur letting Mark keep the earlier Cornish conquests in exchange for his support (or at least neutrality) against the Saxons and Claudas, but you would Arthur would still require at least a nominal pledge of fealty from Cornwall in order to secure Arthur's authority as High King.

The meta angle is that - King Mark and Cornwall are necessary for a few story lines taken from various sources. Mostly Tristam and Isolde. If you don't feel like Mark's independence is justifiable then you can adjust those events. It would actually be a relatively minor footnote to say that Mark swears homage to the High King. (He would definitely fall into the "Poor Vassal" category)

This whole discussion is hypothetical - there is no canon reason, just a base set of circumstances for each gamemaster to justify, accept, or modify as they see fit. Stafford may be the High King, but you maintain sovereignty in your own realm. ;p


And it is a member of the Supreme Collegium (and thus presumably is expected to be included as part of the realm whenever a High King is chosen.)

I could easily be wrong (I have a pretty limited knowledge of the Matter of Britain), but I think the SC is an invention, probably based on similar entities that did exist historically. (The Elector's of the HRE come to mind)

"The council was originally composed of the most distinguished people from all the British tribes and Roman institutions. Eventually, the number of seats was settled at 28, one for each of the Great Cities of Britain. Over time, the right to hold such a seat changed in various places. In some holdings it became the inheritance of one family or was appointed by the local leader, while in others the Legate was still selected from among the leading men of the land."

It's entirely possible that his seat started out somewhere else and drifted to Cornwall through marriage. One of the ironies of the Collegium is that it's entirely dysfunctional and broken.

I think you may also be assuming a stronger degree of federation between the kingdoms than is intended. A high king is elected when needed, to deal problems that threaten all of Britain. Arthur conquers Malahaut and Lothian in "self-defense" so to speak because they won't accept an illegally elected bastard born of rape and adultery as High King are attacking him.