By the start of the 5th century, the Western Roman Empire remained close to its maximum territorial extent, notwithstanding the loss of the "Agri Decumates" during the crisis of the Third Century, but Roman rule had become fragile and many areas were depopulated. In the early years of the century, the Empire withdrew from Great Britain, leaving it open to Anglo-Saxon settlement. Mounting foreign incursions soon resulted in permanent settlement of Germanic and other ethnic groups into territories that became gradually autonomous, were sometimes acknowledged or even encouraged by treaty ("foedus") by the Western Empire, and often embarked on expansion by further conquest.