William Nassau de Zuylestein, 1st Earl of Rochford

William Hendrik of Nassau, Lord of Zuylestein, 1st Earl of Rochford (1649 – 12 July 1708) was a Dutch soldier and diplomat in the service of his cousin William III of England. During the reign of James II of England he travelled to England to liaise with William's English supporters, and played an important part in the preparations of the Glorious Revolution.

He was born at Zuylestein Castle (also spelled Zuylenstein), about twenty miles east of the city of Utrecht. He was the eldest son of Frederick Nassau de Zuylestein, the illegitimate but oldest son of William III's grandfather Prince Frederick Henry and was therefore a half-cousin of William III. His mother was Mary Killigrew, the eldest daughter of Sir William Killigrew. She was a first cousin of Charles II's illegitimate daughter, the Countess of Yarmouth. She moved to the Netherlands in February 1644, aged barely seventeen, as a maid of honour to Mary, princess royal of England and princess of Orange, and married Frederick in 1648. With the dead of his father in 1672, he inherited the Zuylestein Castle and its lands, by which he became known as Lord of Zuylestein (in Dutch: Heer van Zuylestein).

William Henry entered the Dutch cavalry in 1672, but was better known at The Hague for his gallantry and his good looks, and as a companion of the prince. He was trusted by William, and acquitted himself well on a mission of observation to England in August 1687, the nominal purpose being to condole with the queen-consort upon the death of her mother, the Dowager Duchess Laura of Modena.

He was again named envoy in the summer of the following year. His avowed purpose was now to felicitate the Queen Consort, Mary of Modena, on the birth of a prince; his real object to inform himself about the nation and to gauge the probability of James II's summoning a parliament and adopting a more conciliatory policy. He was received by the queen at St. James's on 28 June 1688, and the cordiality of his messages inspired the Queen to write a letter of playful affection to her ‘dear lemon’ (the Princess of Orange); but he wrote at once an account of the sceptical manner in which the birth was received in London, and intrigued with all the prominent malcontents. Clarendon records a number of his movements during July.

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