It finally grew out of the Imperial borders when in 1618 the Hohenzollern electors became dukes of Prussia, then a fief of the Polish Crown, and the lands of Brandenburg-Prussia were ruled in personal union. In 1653, the "Great Elector" Frederick William acquired Farther Pomerania and reached full sovereignty in Ducal Prussia by the 1657 Treaty of Wehlau concluded with the Polish king John II Casimir Vasa. In 1701, Frederick William's son and successor Frederick I reached the consent of Emperor Leopold I to proclaim himself a King "in" Prussia at Königsberg, with respect to the fact that he still held the electoral dignity of Brandenburg and the royal title was only valid in the Prussian lands outside the Empire.[1]