Robert Stewart, 2nd Marquess of Londonderry, (18 June 1769 – 12 August 1822), usually known as Lord Castlereagh, derived from the courtesy title Viscount Castlereagh ( ) by which he was styled from 1796 to 1821, was an Anglo-Irish statesman. As British Foreign Secretary, from 1812 until his suicide in 1822 he was central to the management of the coalition that defeated Napoleon, and he was the principal British diplomat at the Congress of Vienna.