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First marriage

In the castle of Buren on 6 July 1551, the 18-year-old William married Anna van Egmond en Buren, aged 18 and the wealthy heiress to the lands of her father. William thus gained the titles Lord of Egmond and Count of Buren. The couple had a happy marriage and became the parents of three children together; their son Philip William would succeed William as prince. Anna died on 24 March 1558, leaving William much grieved.

A couple of years after Anna's death, William had a brief relationship with Eva Elincx, a commoner, leading to the birth of an illegitimate son, Justinus van Nassau: William officially recognised Justinus as his son and took responsibility for his education – Justinus would become an admiral in adult life.