Philippa Gregory - Why was Warwick called the Kingmaker?

THE KINGMAKER'S DAUGHTER is published on August 16th 2012 in hardback and eBook. pages.simonandschuster.com/kin...
Number One bestselling author Philippa Gregory continues her stunning series, The Cousins' War, with a gripping story of a girl who lost a prince of England; but married a king.
Anne Neville and her sister Isabel are daughters of the most powerful magnate in 15th century England, the Earl of Warwick, nicknamed the 'Kingmaker'. Ever ruthless, always plotting, in the absence of a son and heir, Warwick sets about using his daughters as pawns in his vicious political games.
Anne's life, growing up at the court of Edward IV and his beautiful queen, Elizabeth Woodville, is overturned when her father turns on his former allies, escapes England and invades with an enemy army. Widowed at fourteen, fatherless, with her mother locked in sanctuary, and her sister a vengeful enemy, Anne faces the world alone.
But fortune's wheel turns once again. Anne escapes from her sister's house by marrying the handsome young Duke of Gloucester in secret. But danger still follows her. She finds that she has a mortal enemy in the beautiful queen of England. Anne has to protect herself and her precious only son, from the treacherous royal court, the deadly royal rival, and even from the driving ambition of her husband - Richard III.

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Good explanation! Well done!