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William John Robert Cavendish, Marquess of Hartington (December 10, 1917–September 10, 1944) was the eldest son of Edward William Spencer Cavendish, 10th Duke of Devonshire and the husband of Kathleen Kennedy, sister to American President John F. Kennedy. He was killed in action in Belgium during World War II and his place in the line of succession was taken by his younger brother Andrew Robert Buxton Cavendish.
