The son of an impoverished squire, he became (1665) a page of the duke of York (later James II) and entered (1667) the army. He rose rapidly under York's patronage and c.1678 married Sarah Jennings (see Marlborough, Sarah Churchill, duchess of), attendant and friend of Princess (later Queen) Anne. Under James II he was active in crushing the rebellion (1685) of the duke of Monmouth and was raised to the peerage and made a major general.
Nevertheless, fearing the religious policies of the Roman Catholic king, and concerned about his own career, he corresponded with William of Orange (later William III) and supported him against James in the Glorious Revolution of 1688. He was created earl of Marlborough at William's coronation (1689). Marlborough was successful as a military commander in 1689 and 1690, but William's poor treatment of Anne offended him, and William began to resent Marlborough's ambition and ability. When Marlborough began secret communication with the exiled James II, he was discovered and lost royal favor (1692–98).
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