She ascended the thrones of England, Scotland, and Ireland in 1702 at age 37, Britain's last Stuart monarch. Five years later she united two of her realms, England and Scotland, as a sovereign state, creating the Kingdom of Great Britain. She had a history of personal misfortune, overcoming ill health (she suffered from crippling arthritis; by the time she became queen she was a virtual invalid) and living through 17 miscarriages, stillbirths, and premature births in 17 years. By the end of her comparatively short 12-year reign, Britain had emerged as a great power; the succession of outstanding victories won by her general, John Churchill, the Duke of Marlborough, had humbled France and laid the foundations for Britain's future naval and colonial supremacy.

While the queen's military was performing dazzling exploits on the continent, her own attention - indeed her realm - rested on a more intimate conflict: the female friendship on which her happiness had for decades depended and which became, for her, a source of utter torment.

At the core of Anne Somerset's riveting new biography, published to great acclaim in England, is a portrait of this deeply emotional, complex bond between two very different women: Queen Anne - reserved, stolid, shrewd; and Sarah Churchill, Duchess of Marlborough, wife of the queen's great general - beautiful, willful, outspoken, and whose acerbic wit was equally matched by her fearsome temper.

Against a fraught background, the much-admired historian, author of Elizabeth I , tells the extraordinary story of how Sarah goaded and provoked the queen beyond endurance.