Wilhelm was instrumental in the preparations for the Franco-Dutch War. He went between France and numerous rulers in the Empire to try and 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.