William Cavendish, Marquess of Hartington

Major William John Robert "Billy" Cavendish, Marquess of Hartington (10 December 1917 – 9 September 1944) was an English politician and soldier. He was the elder son of Edward Cavendish, 10th Duke of Devonshire, and his wife, Lady Mary Gascoyne-Cecil. He was the husband of Kathleen Kennedy, sister of the future U.S. President John F. Kennedy.

He was educated at Eton and Trinity College, Cambridge.

Hartington was a member of the Conservative Party. He was selected as the official candidate of the Wartime Coalition for the West Derbyshire by-election on 18 February 1944, in the constituency local to Chatsworth. He was faced by Charles Frederick White, Jr., who resigned from the Labour Party to run as an Independent candidate, evading the Wartime Coalition's ban on partisan campaigning. West Derbyshire had been held by Conservatives since 1923 (Hartington's father and then his uncle by marriage), but in a contentious campaign, White solidly defeated Hartington.

Hartington married socialite Kathleen Kennedy on 6 May 1944 at the Register Office in Chelsea Town Hall on King's Road in London, England. She was the daughter of former U.S. Ambassador to the United Kingdom Joseph Kennedy, and the sister of John, Robert and Ted Kennedy. The Duke of Devonshire and the bride's eldest brother Joseph P. Kennedy Jr., then a lieutenant in the United States Navy, signed the marriage register, and the Duke of Rutland served as best man. Her mother Rose disapproved of the union – the Kennedy family were Roman Catholic and the Dukes of Devonshire were Anglican, and neither would be married in the other's faith.

Four months later, on 9 September 1944, Hartington was killed in action by a sniper in Belgium while serving during World War II as a major in the Coldstream Guards. He was killed one month after his brother-in-law, Joseph Kennedy Jr., was also killed in action.

Hartington's company was trying to capture the town of Heppen, which was being held by troops of the German Waffen-SS.

In the weeks before he died, 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 Somme and 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, Hartington 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."

Hartington is buried at the war cemetery in Leopoldsburg. His place in the order of succession was taken by his younger brother Andrew, who inherited the dukedom six years later.