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Internal disputes

Medieval Brittany was far from being a united nation. The French king maintained envoys in Brittany, alliances contracted by local lords often overlapped and there was no specific Breton unity. For example, Brittany replaced Latin with French as its official language in the 13th century, 300 years before France did so, and the Breton language did not have formal status.

The foreign policy of the Duchy changed many times; the Dukes were usually independent, but they often contracted alliances with England or France depending on who was threatening them at that point. Their support for each nation became very important during the 14th century because the English kings had started to claim the French throne.

The Breton War of Succession, a local episode of the Hundred Years' War, saw the House of Blois, backed by the French, fighting with the House of Montfort, backed by the English. The Montforts won in 1364 and enjoyed a period of total independence until the end of the Hundred Years' War, because France was weakened and stopped sending royal envoys to the Court of Brittany.

English diplomatic failures led to the Breton cavalry commanders Arthur, Comte de Richemont (later to become Arthur III, Duke of Brittany) and his nephew Peter II, Duke of Brittany playing key roles on the French side during the deciding stages of the war (including the battles of Patay, Formigny and Castillon and the Treaty of Arras).

Brittany importantly lost the Mad War against France in 1488, mostly because of its internal divisions that were exacerbated by the corruption at the court of Francis II, Duke of Brittany. Indeed, some rebel Breton lords were fighting on the French side.