The peninsula that became "Brittany" was a centre of ancient megalithic constructions in the Neolithic era. It has been called the "core area" of megalithic culture. It later became the territory of several Celtic tribes, of which the most powerful was the Veneti. After Caesar's conquest of Gaul, the area became known to the Romans as Armorica, from the Celtic term for "coastal area". Its transformation into "Brittany" occurred in the late Roman period, with the establishment of Brythonic settlement in the area. The history behind such an establishment is unclear, but medieval Breton and Welsh sources connect it to a figure known as Conan Meriadoc. Welsh literary sources assert that Conan came to Armorica with the Roman usurper Magnus Maximus, who took his British troops to Gaul to enforce his claims and settled them in Armorica. Regardless of the truth of this story, Brythonic settlement probably increased during the Anglo-Saxon invasion of Britain in the 5th century. Scholars such as Léon Fleuriot have suggested a two-wave model of migration from Britain which saw the emergence of an independent Breton people and established the dominance of the Brythonic (British Celtic) Breton language in Armorica.
In the mid-9th century Nominoe and his successors won a series of victories over the Franks which secured an independent Duchy of Brittany. In the High Middle Ages the Duchy was sometimes allied to England and sometimes to France. The pro-English faction was victorious in 1364 in the Breton War of Succession, but the independent Breton army was eventually defeated by the French in 1488, leading to dynastic union with France following the marriage of Duchess Anne of Brittany to two kings of France in succession. In 1532 the Duchy was incorporated into France.