Count (from 1654 Imperial Prince) of Nassau-Dietz, Stadtholder of Friesland, Groningen and Drenthe. As a second son, William Frederick did not seem destined for the career he eventually would follow. He studied at Leiden University and the University of Groningen and subsequently took a commission in the army of the Dutch Republic, like his male ancestors and his brother. As such he was a junior partner of his future father in law and brother in law William II, Prince of Orange. However, his elder brother died in action near Hulst in 1640. As Henry Casimir was unmarried, and did not have children, William Frederick inherited his titles.
