As part of Armorica since the Gallo-Roman period, Brittany developed an important maritime trade network near the ports of Nantes, Vannes, and Alet, as well as salting factories along its coasts. When Rome encountered crises in the third and fifth centuries, the first wave of island Bretons were asked by the imperial power to help secure their territory, beginning with a migratory movement that was carried out until the sixth century, and saw the beginnings of many kingdoms in the peninsula.