William John Robert Cavendish, Marquess of Hartington (10 December 1917 – 10 September 1944) was the eldest son of Edward Cavendish, 10th Duke of Devonshire and Mary Cavendish, Duchess of Devonshire.He was the husband of Kathleen Agnes Kennedy, sister of future U.S. President John F. Kennedy.

Lord Hartington was a member of the Conservative Party. He stood as the official candidate of the Wartime Coalition in the 18 February 1944 by-election for Derbyshire West. Lord Hartington was defeated by the Independent Charles White who had resigned from the Labour Party to challenge Hartington in contravention of the Wartime Coalition's truce on partisan campaigning.

Lord Hartington was married on 6 May 1944 at the Register Office in Chelsea Town Hall on King's Road in London, England to Kathleen Kennedy the daughter of former US Ambassador to Great Britain Joseph Patrick Kennedy and his wife, Rose Fitgerald Kennedy. She was "given away" by her eldest brother, U.S. Navy Lieutenant Joseph P. Kennedy, Jr.

Four months later, on 10 September 1944, Lord Hartington was killed in action by a sniper in Belgium while serving during World War II as a major in the Coldstream Guards. His company was trying to capture the town of Heppen, which was being held by troops of the German SS.

In the weeks before he died, Lord Hartington's battalion, the 5th, serving in the Guards Armoured Division, had engaged in heavy fighting in Northern France. In early September, they crossed the Sommeand pushed east towards Brussels, where his unit was one of the first to liberate the city.

Of the townsfolk and villagers who turned out and cheered the Allies, and in some cases decorated their tanks, William wrote to his wife of feeling "so unworthy of it all living as I have in reasonable safety and comfort during these years ... I have a permanent lump in my throat and long for you to be here as it is an experience which few can have and which I would love to share with you.

His place in the order of succession was taken by his younger brother, Lord Andrew Cavendish — later 11th Duke of Devonshire.

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