The first historian to look at this thorny subject was Dom Taillandier in the 18th century.
You only have to look at a map to follow the implantation of the Bretons "invaders" in Armorica. The dominance of Bretons over the Gauls can be seen in village and place names, with prefixes plou, tre or lan.
In the 6th century, three kingdoms, reigned by sovereigns from father to son, formed Brittany :
The Francs, calling themselves the heirs of Roman organisation, considered these rulers to be counts and not kings. At the same time, they refused Brittany's independence. A succession of conflicts and wars followed, waged by Childebert (Clovis' son) to Pépin le Bref.
The Bretons were disunited, and slowly but surely the Francs extended their domination, without however pacifying the native people.
Governed by palace officers, or Marquis mandated for the purpose, Brittany regularly revolted. At the death of Charlemagne in 814, his son Louis (le débonnaire) succeeded him. Weary of the Bretons' treachery, he decided to defeat their king, Murman. A 4-year guerrilla war began and in 818, during a new campaign by the Francs against the Bretons, Murman was killed, perhaps in Piziac.
In 822, a new revolt by the Bretons took place, with their commander Guihomarch. Beaten 2 years later, he pledged allegiance to Louis le Pieux, before retaking arms, and being killed in 825. On February 9, 833, Louis le Pieux changed strategy and named a Breton Count of Vannes, Nominoë.
In 840, with the death of Louis le Pieux, Charles le Chauve became king.
In 843, Nominoë revolted against Charles, and captured Nantes, in reply to the attack on Messac (his place of residence) by Renaud, Count of Nantes. That year also saw a raid on the town by Normans, which remains in collective memory as the worst massacre the town has ever seen, the Bretons having left the town without surveillance. And the Francs continued to wage war.
845 was an important date for Brittany, as Nominoë beat Charles le Chauve in the marshes of Ballon, near Redon. In August 851, Erispoë, Nominoë's son, beat Charles in Jengland to the north of Nantes, and in September, the two men met at Angers. Charles gave Erispoë the royal insignia to the lands of his father (Rennes, Nantes, and the Retz region south of the Loire).
Erispoë became King of the Bretons, whilst accepting to be King Charles' vassal.
However, fledgling Brittany saw its coast colonised by Vikings and Normans and we have to wait for Alain Barbetorte, Count of Cornouaille and Nantes at the end of the 10th and beginning of the 11th centuries, for the invaders to be ejected from Brittany in 939.
11th - 13th centuries: Breton power was diluted amongst a multitude of families and England and France took the opportunity to recover sovereign power.
15th century: on January 8, 1499, Anne, Duchess of Brittany, married King Louis XII in the chapel of her chateau at Nantes, and thus attached the Duchy of Brittany to the Kingdom of France.
Documentary source: Histoire de la Bretagne (History of Brittany) by Joël Cornette, Editions Points 2008