He made a treaty with the King of France and married Jeanne d'Albret, and with this backup dared to challenge the Emperor.

In accordance with the Treaty of Venlo (1543) that was the result of this war, Guelders and the County of Zutphen were transferred to Charles V, Holy Roman Emperor, combining them with the Habsburg Netherlands.

The three duchies all got new main fortresses as major strongpoints, for the older medieval fortifications had proved to be no match against the Imperial artillery.

For this task, the renowned Italian architect Alessandro Pasqualini from Bologna was hired, who had already made some impressive display of his craft in the Netherlands.

[2] William married Maria of Austria (1531–1581), daughter of Ferdinand I, Holy Roman Emperor,[2] and Anna of Bohemia and Hungary, on 18 July 1546 and they had the following children: