Willem Frederik (William Frederick), Prince of Nassau-Dietz, was the grandson of Johann VI, Count of Nassau-Dillenburg[KD1] , the brother of William of Orange. When his older brother Hendrik (Henry) Casimir I died childless Willem Frederik became stadtholder of Friesland. Groningen and Drenthe chose Frederik Hendrik (Frederick Henry), who was also stadholder of the other Dutch provinces. For many years Willem Frederik was in conflict with Frederik Hendrik, who also aspired the Frisian stadtholdership.
Despite the problems with Frederik Hendrik, Willem Frederik was keen to marry one of Frederik Hendrik’s daughters. After a great deal of resistance from his future mother-in-law Amalia of Solms-Braunfels, he married Albertine Agnes. They had two daughters and a son. The youngest daughter died at an early age. Willem Frederik was killed when he accidentally shot himself in the jaw while cleaning his pistol.