Henry Sydney, 1st Earl of Romney in the context of Master-General of the Ordnance

The Invitation to William was a letter sent by seven Englishmen (six nobles and a bishop), later referred to as "the Immortal Seven", to stadtholder William III, Prince of Orange, dated 30 June 1688 (Julian calendar, 10 July Gregorian calendar). In England, the heir apparent to the throne, James Francis Edward Stuart, had just been born to the unpopular King James II of England, and baptised a Catholic. The letter asked William, who was a nephew and son-in-law of James II, to use military intervention to force the King to make his eldest daughter, Mary, William's Protestant wife, his heir. The letter alleged that the newborn prince was an impostor.

The letter informed William that if he were to land in England with a small army, the signatories and their allies would rise up and support him. The Invitation briefly rehashed the grievances against King James. It claimed that the King's son was supposititious (fraudulently substituted) and that the English people generally believed him to be so. The present consensus among historians is that he was almost certainly their real son. The letter deplored that William had sent a letter to James congratulating him for the birth of his son, and offered some brief strategy on the logistics of the proposed landing of troops. It was carried to William in The Hague by Rear Admiral Arthur Herbert (the later Lord Torrington) disguised as a common sailor, and identified by a secret code.