Pliny the Elder, in his Natural History (2.17.105), claims that Armorica was the older name for Aquitania, stating Armorica's southern boundary extended to the Pyrenees. Taking into account the Gaulish origin of the name, this is perfectly correct and logical, as Aremorica is not a 'country name', but a word that describes a type of geographical region - a region that is by the sea. Pliny lists the following Celtic tribes as living in the peninsula: the Aedui and Carnuteni as having treaties with Rome; the Neldi and Secusiani as having some measure of independence; and the Boii, Senones, Aulerci (both the Eburovices and Cenomani), the Parisii, Tricases, Andicavi, Viducasses, Bodiocasses, Veneti, Coriosvelites, Diablinti, Rhedones, Turones, and the Atseui.
Trade between Armorica and Britain, described by Diodorus Siculus and implied by Pliny was long-established. Because, even after the campaign of Crassus in 57 BC, continued resistance to Roman rule in Armorica was still being supported by Celtic aristocrats in Britain, Julius Caesar led two invasions of Britain in 55 and 54 in response. Some hint of the complicated cultural web that bound Armorica and the Britanniae (the "Britains" of Pliny) is given by Caesar when he describes Diviciacus of the Suessiones, as "the most powerful ruler in the whole of Gaul, who had control not only over a large area of this region but also of Britain (De Bello Gallico ii.4). Archaeological sites along the south coast of England, notably at Hengistbury Head, show connections with Armorica as far east as the Solent. This 'prehistoric' connection of Cornwall and Brittany set the stage for the linked that continued into the medieval era. Still farther East, however, the typical Continental connections of the Britannic coast were with the lower Seine valley instead.
Archeology has not yet been as enlightening in Iron-Age Armorica as the coinage, which has been surveyed by Philip de Jersey..
Under the Roman Empire, Armorica was administered as part of the province of Gallia Lugdunensis, which had its capital in Lugdunum, (modern day Lyons). When the Roman provinces were reorganized in the 4th century, Armorica was placed under the second and third divisions of Lugdunensis. After the legions retreated from Britannia (407) the local elite there expelled the civilian magistrates in the following year; Armorica too rebelled in the 430s and again in the 440s, throwing out the ruling officials, as the Romano-Britons had done. At the Battle of the Catalaunian Plains in 451 a Roman coalition led by General Flavius Aetius and the Visigothic King Theodoric I clashed violently with the Hunnic alliance commanded by King Attila the Hun. Jordanes lists Aëtius' allies as including Armoricans and other Celtic or German tribes (Getica 36.191).
The "Brittany" peninsula came to be settled with Britons from Britain during the poorly documented period of the 5th-7th centuries. These settlers, whether refugees or not, made their presence felt in the naming of the westernmost, Atlantic-facing provinces of Armorica, Cornouaille ("Cornwall") and Domnonea ("Devon"). These settlements are associated with leaders like Saints Samson of Dol and Pol Aurelian, among the "founder saints" of Brittany.
Questions of the relations between the Celtic languages of Britain— Cornish and Welsh— and Celtic Breton are far from settled. Martin Henig (review, 2003) suggests that in Armorica as in sub-Roman Britain, "there was a fair amount of creation of identity in the migration period. We know that the mixed, but largely British and Frankish population of Kent repackaged themselves as 'Jutes', and the largely British populations in the lands east of Dumnonia (Devon and Cornwall) seem to have ended up as 'West Saxons'. In western Armorica the small elite which managed to impose an identity on the population happened to be British rather than 'Gallo-Roman' in origin, so they became Bretons. The process may have been essentially the same." this flux of shifting self-identification in the Early Middle Ages, in the modern view that is supplementing traditional assertions of continuity from the Iron Age. When Vikings or Northmen settled in the Cotentin peninsula and the lower Seine around Rouen in the ninth and early tenth centuries, and these regions came to be known as Normandy, the name Armorica fell out of use.