Based on a personal vassalic link between the emperor and Nominoë, the rebellion of his sons led enough troubles in the peninsula that Louis had to led another campaign in 837, taking hostage to keep Bretons loyal enough either not to participate in the whole mess or not to make opportunistic raids; troubles that reached their high point in 840 and the death of Louis.

The personal vassalic link between Nominoe and Louis being broken down, and the grandsons of Charlemagne still fighting each other, the former didn't took sides, and firstly pledged himself to Charles the Bald, supported by the local Frankish aristocracy who tied links with the Bretons into transfrontalieers networks. The same aristocratic families, and primarily the Lambert, that involved Bretons into their own wars against Charles, frustrated not obtaining the honors, titles and lands they felt were owned to us : not two years later having acknowledged the king of western Francia as suzerain, Nominoë is at war with him and inflict a great defeat in Messac being like bread given to Vikings that utterly plunder Nantes without protection soon thereafter, a threat that'd keep Bretons from campaiging too far, but did not prevent them to defeat Charles anew.

This allowed Nomione to negotiate his position in a better place ; being named 'dux' of the Bretons, acknowledging his (at least claimed) authority over all the peninsula not under direct Frankish control and under a very technical royal suzerainity : an authority further reinforced by his victories against Normans and by establishing a 'Breton' synod where bishops faithful to Charles were replaced, leading to a pre-emptive attack against Charles where Rennes and Nantes were taken, Bretons raiding further east until the duke died, his son Ersipoë succeeding him and defeating Charles in 851, obtaining out of the peace not only the lands taken by his father, but against acknowledgement of Charles' suzerainty, a royal distinction. This is not the first time a sub-kingdom was made in Carolingian Francia, and not even the first time under duress : but Aquitaine, Italy, Lotharingia, etc. all were attributed to a member of their dynasty. Ersipoë, not only furthering the authority of his father in Brittany, giving himself the means of regalian apparatus and decisions, and while being a part in the broad Frankish ensemble, firmly forced this evolution to be acknowledged by Franks.

But this sub-kingdom of Brittany is still precarious, under the same threat of Viking raids and nobiliar susceptibility than the western kingdom of Franks the new king have all interest still search the protection : Charles seem to have played a double game there, both promising Salomon (a cousin of Ersipoe) a 'third of Brittany' as a royal duke and obtained in marriage for his son the daughter of Ersipoe. The Carolingian king, as purveyor of honors and legitimacy, in intervening into aristocratic networks, remains a key figure in IXth century politics.

The situation was soon resolved by the murder of Ersipoe by Salomon, with the aposteriori acceptance of Charles making the point clear to Solomon, trough his bishops, he's to respect the subordinate position that is supposed to be his. The new ruler (not yet king, and using broad term as "ruler" or "prince") retaliates by allying with his ennemies in Aquitaine and Neustria (first Robertians, then Rogronides when the first would came to dominate the region) and even briefly Charles' son Louis : Solomon's strength and prestige is enough, indeed, to support and fuel nobiliar resistance against Charles the Bald, whoever they could be. From this, he gained more land eastwards and was excplicitely exempted from tribute, making his vassality thinner as it ever was.

This shouldn't be understood as a Breton takeover of Franks, but as the emergence of a Breton principality along Frankish lines : if Breton society remains largely distinct with its own institutions, the counts, dukes and kings of Brittany ruled along the parameters of the Frankish world. Especially interesting is the opposition we see between the machtierns (Breton elites, possibly of royal origin and at least aristocratic, with legal, fiscal and social power over local people), searching either to control or associate them (for instance, promoting them as counts) or to diminish their influence while using all the apparatus and dignitary of Carolingian kings for their own use : missi, vassality, synods and general assemblies. This is even more true, obviously, of the new territories they gained where Frankish aristocracy remained influential and in charge.

Solomon himself had to deal with more than Franks, Robertians or Carolingians, with the continued raiding of Normans : after having sporadically allied with them against other Vikings or Franks, Charles and Solomon came to an agreement. Bretons recieved the westernmost parts of Neustria, in order to defend (and more exactly reconquer) the region against Vikings raids as efficiently they beaten Franks, soon officially acknowledging the plenitude of the royal title of Solomon. Altough victories would be rare, and the campaign more about bribing Vikings away, it was enough to cement his prestige as "prince of all Brittany and a great part of Gaul"

This evolution wasn't accepted by all, and Solomon ended up being blinded then murdered himself by a conspiracy of counts, nobles and even (in a Carolingian fashion) by his own sons. As the co-conspirators were seemingly unified mostly in their opposition to Solomon but had no lasting agreement on how to split the kingdom, it soon collapsed into civil war and further weakened and damaged by consecutive Vikings raids and establishment (notably in Nantes), that in spite of a brief respite thirty year after Solomon's death under Alan I, threatened to make the peninsula a second Normandy, so to speak. The royal title and prestige would barely survive the first civil war, and would disappear in the succession crisis following Alan I death, the descendents of Erispoë and Solomon adopting a ducal title from the Late Carolingian kings of France in order to strengthen their fragile legitimacy. But that's a story for another time.