Robert Stewart, Viscount Castlereagh
(1769–1822), briefly Marquess of Londonderry after his father's death in 1821, was a British/Irish politician. He was a member of the Irish House of Commons from 1790, and of the British House of Commons from 1794. He was instrumental in getting the Irish Act of Union passed in both parliaments, and was a member of the new United Kingdom
parliament from 1801. He joined the cabinet in 1802, and in 1809 fought a duel with pistols with another cabinet member, George Canning. As Foreign Secretary, Castlereagh took a leading role at the Congress of Vienna in 1815, which reshaped the map of Europe after the fall of Napoleon.
Castlereagh became unpopular because of the repressive policy of the Conservative government, and suffered a nervous breakdown. In August 1822 he told the King (George IV) that he was being blackmailed for "the same crime as the Bishop of Clogher" – in other words, homosexuality. A few days later he committed suicide by cutting his throat with a penknife.