The old kingdom of Cornouaille became a county with vassal barons at Pont l'Abbé, Pont Croix, the abbot of Landevennec, and the viscount of Le Faou. In the interior were the viscounts of Poher and Gourin.
The old kingdom of Domnonia was divided into three counties, Léon, Penthièvre and Tréguier.
The Ducal crown did not long remain in the family of Alan Barbetorte. After internecine war lasting forty years, Conan, Count of Rennes, assumed the title (990), and the dukes of his house spent their time in fighting and crushing their own kinsmen. Geoffrey I. had married a Norman wife, and he had by her two sons, Alan and Eudo. In 1034 Eudo, jealous and ambitious, demanded of his brother a share in the duchy. Alan gave him the counties of Tréguier and Penthièvre, and thus Eudo became the ancestor of that great and dangerous family of Penthièvre, which maintained undying rivalry with the ducal house, and made of Brittany a field of civil war for centuries. Conan II. succeeded as a child of three months, and his uncle ruled in his name, aided by the Normans. When Conan came of age, he had to fight against Eudo; he invaded Normandy, but was cut off by poison. When William the Conqueror became King of England, Brittany was nipped between France and Normandy, and became an object of ambition to both, and a common battlefield.
For five hundred years this continued. Brittany writhed and strove for her independence, and had no desire to become either a province of France or an English colony. The war broke out under Duke Hoel in 1076 when he invoked the aid of Philip I. against William the Conqueror. However, under Alan Fergant and Conan III. the land had rest for eighty years, and then the trouble began again with renewed violence. Conan's death in 1148 gave rise to a war of succession that lasted eight years. Conan IV. assisted by the English succeeded in establishing himself in the ducal seat, and he favoured the English in every way. Henry II. of England married his son Geoffrey Plantagenet to Constance, daughter of Conan IV., the heiress of Brittany, and Geoffrey was crowned at Rennes in 1169. This was of advantage so far that it introduced Norman civilisation into a duchy that was backward and barbarous. The churches built in the 12th century were erected by architects of the Norman and French schools. Such are the cathedrals of S. Pol-de-Léon and S. Malo and the churches of Guérande. Geoffrey died in 1187, and his son Arthur fell into the hands of his uncle, King John, who had him murdered at Rouen (1203). Constance did not die broken-hearted and despairing, as represented by Shakespeare, but married Guy de Thouars, and had by him a daughter and heiress, who was married to Pierre de Dreux.
We may pass over the ensuing history till we reach John III. who died in 1341, without issue, and who, hating his half-brother, Jean de Montfort, bequeathed the succession to his niece Jeanne de Penthièvre, whom he married to Charles de Blois, nephew of Philip VI. of France. This was the signal for the outbreak of the terrible and desolating War of the Succession of the two Jeannes. In it, neither of those most interested were for the most part of the time leaders of their hosts. At the outset Jean de Montfort was taken prisoner (1342), and was kept in prison till his health was broken, and he was discharged only to die (1345). But his intrepid wife Jeanne of Flanders carried on the conflict. At the Battle of La Roche-Derrien (1347) Charles of Blois was captured and conveyed a prisoner to England, and the conduct of the war fell to his wife Jeanne. The English espoused the side of Montfort, and the French that of Charles of Blois. The success of the battle of La Roche was followed by the signal victory of Mauron (1352). The war dragged on, and Charles was released in 1356, to renew the contest with fresh cruelty. He had now as his best assistant Bertrand du Guesclin, an heroic and honourable soldier, and one of the best captains France has produced. But in the decisive battle of Auray (1364) Charles was killed, and Du Guesclin taken prisoner. A few months later, Jean de Montfort the younger was recognised duke under the title of John IV. But the war was not at an end. Now that Charles was dead, the Bretons of Penthièvre rallied about Oliver de Clisson, and the old strife continued under other names.
The country was ravaged by Companies, under commanders who passed from one side to the other as suited their convenience. John IV. attempted to have Clisson assassinated in Paris (1392). The attempt failed, and served only to exasperate Clisson and aggravate the war. It was resolved into a family vendetta. In 1420 Oliver de Clisson, grandson of Charles de Blois, and of Oliver, treacherously obtained possession of John V. and imprisoned him. A war ensued, and before the duke could be liberated, much blood was shed; as the cause of the Penthièvre family was not, on this occasion, espoused by France, it was crushed and the apanage of Penthièvre was confiscated.
Francis I. (1442-50) conceived an animosity against his brother Gilles de Bretagne whom he accused of favouring the English. He delivered him over to his mortal enemy, who starved the unhappy prince to death. Pierre II. succeeded, but as he died without issue, as well as Francis, the succession passed to Arthur of Richmond their uncle. He was succeeded by Francis II. who died in 1488, leaving an heiress, Anne, who married first Charles VIII. of France (1491), and on the death of Charles (1498) married Louis XII., and thus, the duchy was finally united to the crown of France.
The Reformation made no way with the people of Brittany, but was embraced by the Rohan, the Rieux, the Laval, and other noble lords, who coveted the estates of the Church. The chateaux of Blain and Vitré were for a while centres of Huguenot propaganda in Brittany. The province would, however, have remained at peace, but that its governor, the Duke de Mercœur was a devoted adherent to the house of Guise, and he proposed to make of Brittany a stronghold of the League. When Henry IV. came to the throne in 1589, he was a Calvinist. There were three parties in Brittany mutually antagonistic, the Leaguers supported from Spain, the Huguenots and the Royalists. The city of Rennes, without abandoning the Faith remained true to Henry IV. Nantes became the headquarters of the League. The Huguenots, from Vitré, and the castles of the family of Rohan, swept the country ravaging and burning. Nine years of war ensued between 1589 and 1598. A swarm of brigands placed themselves under the flag of the League, or of the King or of the Bible, and wrought intolerable misery. Moreover, the peasants, maddened by their sufferings, rose against all alike, besieged the castles indiscriminately and massacred every man in harness. Brittany was almost depopulated, and wolves preyed on human corpses in the open day. One of the worst ruffians of this period was Fontenelle, a cadet of the Breton family of Beaumanoir. He sacked Roscoff, Carhaix, and ravaged the diocese of Tréguier. But his worst atrocities were committed at Pont l'Abbé and Penmarch, which was once a flourishing town rivalling Nantes, but which has never recovered the butcheries there committed by Fontenelle, and its ruined houses have never been rebuilt. The atrocities committed by him at Pont l'Abbé defy description. He delighted in seating his victims on iron chairs and broiling them to death, or in immersing them in mid-winter in vats of ice-cold water, and thus leaving them to perish in dungeons. In some parishes visited by him, where the population had numbered a thousand adults, he reduced it to twelve. To the miseries produced by civil war succeeded a Black Death, which almost completed the depopulation. Fontenelle was taken in 1598, but pardoned; he was arrested for fresh crimes in 1602, and slowly tortured to death.
The province remained in peace till 1675, when taxation became so burdensome, that the people rose in insurrection. It was put down with great barbarity.