In 1672 the Emperor joined the Franco-Dutch War as an ally of the Dutch, finding himself opposed by several bishops in the northwest part of the Empire. The Fürstenberg brothers being regarded by the Imperialists as the main cause of this disaster, Wilhelm was arrested for treason on 14 February 1674 by imperial cavalry after chasing his carriage through Cologne. At the time of his arrest he was making ninety thousand livres a year from France. He was hurried off to Vienna and was tried for his life. He was saved by the intervention of the papal nuncio at Cologne, but was kept in prison until the signature of the Treaty of Nijmwegen in 1679.