Robert Stewart Viscount Castlereagh
 Robert Stewart 2nd Marquess of Londonderry KG GCH PC PC (Ire) (18 June 1769 – 12 August 1822) usually known as Lord Castlereagh (/ˈkɑːsəlreɪ/ KAR-sul-RAY) was an Irish and British statesman. As British Foreign Secretary from 1812 he was central to the management of the coalition that defeated Napoleon and was the principal British diplomat at the Congress of Vienna.