International negotiations
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 to 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.
When Philip IV of Spain died in 1665, the great powers wondered how long his heir, the sickly Charles II, would last. Wilhelm was put to work by France to begin negotiations related to the succession in Spain and in the Spanish Netherlands. In the run up to the War of Devolution, Wilhelm carried funds to Maximilian to fund the build-up of an army for Cologne. He also visited the Count Palatine of Neuburg, Philip Wilhelm, and the Elector of Bavaria to persuade them not to allow Austrian forces to move through their lands to oppose French maneuvers in the Netherlands. On 8 January 1667, Wilhelm went to Vienna representing Cologne (but acting on behalf of France) to sound out the Emperor's feelings about the succession, perhaps splitting the Netherlands from Spain after the death of Charles. There he spent much time establishing relationships with various ministers at the court. He was offered titles and money to join the Habsburg side and work for the end of the League of the Rhine, but he demurred. He was also sought by Petar Zrinski, who wished his help (and thus French help) in a Hungarian revolution. Wilhelm avoided being seen too often with him. However, he did let Louis XIV know of the Hungarian revolutionaries, and their potential to provide an opportune diversion for Austrian forces. This laid the ground for a 1668 treaty between France and Austria agreeing that if Charles died without an heir, France would receive the Spanish Netherlands, the County of Burgundy, Navarre, the Philippines, the Two Sicilies, and Morocco, while the remaining Spanish territories would go to Austria.
In early 1668, Wilhelm was negotiating with Grand Pensionary Johan de Witt to try to come to an arrangement in the War of Devolution to partition the Spanish Netherlands between France and the Dutch Republic.
Early in 1669, he spent a month training with his French regiment in Lille. He had been appointed colonel of this regiment shortly before. This appointment made it even harder for others to believe he was not working directly for Louis XIV. However, Wilhelm was not above taking advantage of the French for personal gain. That July, he had his brother, Hermann Egon, arrange a meeting so that it would appear the Dutch were negotiating trade agreements with the League of the Rhine against France. This was done so that the French would give the brothers money that they would then use to purchase Mainau.
Wilhelm was instrumental in the preparations for the Franco-Dutch War. He went between France and numerous rulers in the Empire to try to build an unbeatable alliance for the attack on the Dutch. In 1670 he secured French support for the army of Cologne, guaranteeing the interests of Maximilian in the treaty. He then went to convince Frederick William, Elector of Brandenburg, to join the offensive alliance, though he was ultimately unsuccessful. By July 1671, Wilhelm convinced the Bishop of Münster to join the French alliance.
