Having become ruler of Brandenburg and Prussia by his father's death in December 1640, Frederick William set to work at once to repair the extensive damage wrought during the Thirty Years' War, still in progress. After some difficulty he secured his investiture as duke of Prussia from Wladislaus, king of Poland, in October 1641, but was not equally successful in crushing the independent tendencies of the estates of Cleves. It was in Brandenburg, however, that he showed his supreme skill as a diplomatist and administrator. His disorderly troops were replaced by an efficient and disciplined force; his patience and perseverance freed his dominions from the Swedish soldiers; and the restoration of law and order was followed by a revival of trade and an increase of material prosperity. After a tedious struggle he succeeded in centralizing the administration, and controlling and increasing the revenue, while no department of public life escaped his sedulous care (see BRANDENBURG). The area of his dominions was largely increased at the peace of Westphalia in 1648, and this treaty and the treaty of Oliva in 1660 alike added to his power and prestige. By a clever but unscrupulous use of his intermediate position between Sweden and Poland he procured his recognition as independent duke of Prussia from both powers, and eventually succeeded in crushing the stubborn and lengthened opposition which was offered to his authority by the estates of the duchy (see PRUSSIA). After two checks he made his position respected in Cleves, and in 1666 his title to Cleves, Juelich and Ravensberg was definitely recognized. His efforts, however, to annex the western part of the duchy of Pomerania, which he had conquered from the Swedes, failed owing to the insistence of Louis XIV. at the treaty of St Germain-en-Laye in 1679, and he was unable to obtain the Silesian duchies of Liegnitz, Brieg and Wohlau from the emperor Leopold I. after they had been left without a ruler in 1675.