In 1659, Cardinal Mazarin asked Maximilian and Johann Philipp von Schönborn, the Elector of Mainz, to oversee negotiations to end the Franco-Spanish War. Maximilian thereafter sent Wilhelm regularly to the French court, to negotiations and ultimately to the signing of the Treaty of the Pyrenees. In 1661, Wilhelm spent months in Paris conveying the suit of Charles IV, Duke of Lorraine, who wished to marry Anne Marie Louise d'Orléans, Duchess of Montpensier. While she rejected the offer, she did enjoy spending time with Wilhelm, admiring his intelligence and his knowledge of foreign affairs. In 1664, Leopold elevated the territory of Fürstenberg-Heligenberg from a county to a principality, and also extended the title of Prince to Franz and Wilhelm, though their brother ruled the territory. This was done to try and draw the brothers closer to Habsburg Austria, but did not appear to change much. As the representative from Cologne, Wilhelm drove the negotiations in 1665 and 1666 to end the involvement of Bernhard von Galen, the Bishop of Münster, in the Second Anglo-Dutch War so that they would not overly restrict the authority of the Bishop of Münster to wage war in the future.