The Thirty Years' War between 1618 and 1648 devastated Berlin. One third of its houses were damaged or destroyed, and the city lost half of its population. Frederick William, known as the "Great Elector", who had succeeded his father George William as ruler in 1640, initiated a policy of promoting immigration and religious tolerance. With the Edict of Potsdam in 1685, Frederick William offered asylum to the French Huguenots.