The Islamic conquest was only very slowly undone, over the course of seven centuries in what the Christians of Spain called the "Reconquista". Three main forces were involved in this process, the Visigothic holdouts in the Asturias, the holdouts in Navarre and the Pyrenees, and the Franks of Aquitaine. The Reconquista, as a concerted effort to remove the Muslims from the territories they held, commenced in the reign of Alfonso I (739 – 757). Alfonso led an offensive into the valley of the Duero and left the region depopulated, the so-called "Desert of the Duero". For the next century, this prevented any serious Islamic incursions into the Christian territories of the north. During the late 8th and early 9th centuries, the Franks under their Carolingian rulers took up the cause of reconquest along the Mediterranean littoral. By 797, Charlemagne's son, Louis the Pious, captured Barcelona, establishing a clear bulwark against future invasions. The Basques extended their kingdom as far as Nájera, and a widespread "repoblación" of the depopulated areas began, extending Christian borders southwards.