Robert Stewart, Viscount Castlereagh

Robert Stewart, 2nd Marquess of Londonderry (18 June 1769 – 12 August 1822), usually known as Lord Castlereagh, derived from the courtesy title Viscount Castlereagh (UK: /ˈkɑːsəlreɪ/ KAH-səl-ray) by which he was styled from 1796 to 1821, was an Irish-born British statesman and politician. As secretary to the Viceroy in Ireland, he worked to suppress the Rebellion of 1798 and to secure passage in 1800 of the Irish Act of Union. As the Foreign Secretary of the United Kingdom from 1812, he was central to the management of the coalition that defeated Napoleon, and was British plenipotentiary at the Congress of Vienna. In the post-war government of Lord Liverpool, Castlereagh was seen to support harsh measures against agitation for reform, and he ended his life an isolated and unpopular figure.

Early in his career in Ireland, and following a visit to revolutionary France, Castlereagh recoiled from the democratic politics of his Presbyterian constituents in Ulster. Crossing the floor of the Irish House of Commons in support of the government, he took a leading role in detaining members of the republican conspiracy, the United Irishmen, his former political associates among them. After the 1798 Rebellion, as Chief Secretary for Ireland he pushed the Act of Union through the Irish Parliament. However, unable to overcome the resistance of King George III to the Catholic Emancipation that they believed should have accompanied the creation of a United Kingdom, both he and Prime Minister William Pitt resigned.

From 1805 Castlereagh served as Secretary of State for War in Pitt's second administration and, from 1806, under the Duke of Portland. In 1809 he was obliged to resign after fighting a duel with the foreign secretary, George Canning. In 1812, Castlereagh returned to government serving Lord Liverpool as Foreign Secretary and as Leader of the House of Commons.

Castlereagh organised and financed the alliance that defeated Napoleon, bringing the powers together at the Treaty of Chaumont in 1814. After Napoleon's second abdication in 1815, Castlereagh worked with the European courts represented at the Congress of Vienna to frame the territorial, and broadly conservative, continental order that was to hold until mid-century. He blocked harsh terms against France believing that a treaty based on vengeance and retaliation would upset a necessary balance of powers. France restored the Bourbon kings and its frontiers were restored to 1791 lines. Its British-occupied colonies were returned. In 1820, Castlereagh enunciated a policy of non-intervention, proposing that Britain hold herself aloof from continental affairs.

After 1815, at home, Castlereagh supported repressive measures that linked him in public opinion to the Peterloo Massacre of 1819. Widely reviled in both Ireland and Great Britain, overworked, and personally distressed, Castlereagh died by suicide in 1822.