Normandy was a duchy in the North-West of what later became France under the Ancien Régime which lasted until the later part of the 18th century. Initially populated by Celtic tribes in the West and Belgic tribes in the North East, it was conquered in AD 98 by the Romans and integrated into the province of Gallia Lugdunensis by Augustus. In the 4th century, Gratian divided the province into the civitates that constitute the historical borders. After the fall of Rome in the 5th century, the Franks became the dominant ethnic group in the area and built several monasteries. Towards the end of the 9th century, Viking raids devastated the region, prompting the establishment of the Duchy of Normandy in 911. After 150 years of expansion, the borders of Normandy reached relative stability. These old borders roughly correspond to the present borders of Lower Normandy, Upper Normandy and the Channel Islands.