Normandy was a province in the North-West of France under the Ancien Régime which lasted until the latter part of the 18th century. Initially populated by Celtic tribes in the West and Belgic tribes in the North East, it was conquered in 98 AD 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, built several monasteries, and replaced the barbarism of the region with the civilization of the Carolingian Empire. Towards the end of the 8th 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. Mainland Normandy was integrated into the Kingdom of France in 1204. The region was badly damaged during the Hundred Years War and the Wars of Religion, the Normans having more converts to Protestantism than other peoples of France. In the 20th century, D-Day, the 1944 Allied invasion of Western Europe, started in Normandy. In 1956, mainland Normandy was separated into two régions, Lower Normandy and Upper Normandy, and were reunified in 2016.