born June 18, 1769, Dublin, Ire.
died Aug. 12, 1822, London, Eng.
British politician.
He was elected to the Irish Parliament in 1790 and later served in the British Parliament (1794–1805, 1806–22). As chief secretary for Ireland (1798–1801), Castlereagh singlehandedly forced the Act of Union through the Irish Parliament in 1800. He served as Britain's secretary for war (1805–06, 1807–09) and as secretary for foreign affairs and leader of the House of Commons (1812–22). Considered one of the most distinguished foreign secretaries in British history, he played a leading role in bringing together the Grand Alliance that overthrew Napoleon and in deciding the form of the peace settlements at the Congress of Vienna . Beset with paranoia and believing that he was being blackmailed, he eventually committed suicide.