William III of the house of Orange-Nassau was the Prince of Orange, a principality in what is today southeastern France, and Stadholder (head of state) of several provinces in the Dutch Republic. He became the effective ruler of the Dutch Republic during the Third Anglo-Dutch War in 1672, when the combined forces of England and France threatened to overrun Holland. William led the military in the field while the civilian government in the Hague crumbled; in order to prevent an overland invasion of Holland, William ordered dikes broken and substantial amounts of territory that had been reclaimed from the sea were flooded. A lifelong adversary of Louis XIV of France, William married the sister of the then Duke of York, the future James II Stuart, to secure a claim to the throne of England.

William's ambitions were realized in the Glorious Revolution of 1688, in which resistance to the Catholic King James II reached a fever pitch after James' wife Mary of Modena gave birth to a son. Faced with the possibility of a hereditary monarch weilding absolute power and subscribing to the Catholic faith, English nobles cast about for a Protestant with a legitimate claim to the throne. Mary Stuart had been next in line, and William and Mary arranged to be invited to come to England and present their claims to the throne. Upon their landing, James II sent an army out to fight them, but the army took its cue from its charismatic leader John Churchill and switched allegiance to William. James II then fled the country. Parliament met and voted that James had abdicated and awarded the crown to William and Mary jointly and to concede William's right to continue ruling England in the event that Mary Stuart predeceased him (which, in fact, she did). In settlement of the claims, William agreed to certain limitations on the power of the monarch, ending the era of absolute monarchy and setting in motion a chain of political and social events which would by the twentieth century lead to the English monarchy becoming almost completely ceremonial in nature.

In the Cycle, Daniel Waterhouse is an instrumental figure in the Glorious Revolution, gathering the invitations to William to assume power in England, personally delivering them to William III in the Hague accompanied by his rival William Penn. Penn and Daniel become reluctant allies because of this endeavor.

William of Orange is depicted in the Baroque Cycle as cynical, calculating, and ruthless, particularly in his initial appearance and the manner in which he secures Eliza's loyalty and services as a spy. However, he eventually is proven to be the strong, sure hand that England needs to resist falling in to the political orbit of France under the formidable Louis XIV. He is also portrayed as bisexual and in stern command of the manner in which his carnal desires are expressed so as to never cede power or expose himself to vulnerability to any favored courtier. He is also shown to be a fan of the sport of "sand-sailing" involving the use of sail power on a light wheeled vehicle along the beach.

Despite his maintenance of a mistress and apparent affairs with men of his court, William appeared to genuinely love his wife Mary Stuart, and was psychologically shaken by her death. When she did die, William's political support in England began to erode. He and Mary had no issue, and after his death in 1702 he was succeeded by his sister-in-law Anne Stuart, the last of the Stuart monarchs.

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