Robert Stewart, 2nd Marquess of Londonderry,, usually known as Lord Castlereagh, derived from the courtesy title Viscount Castlereagh by which he was styled from 1796 to 1821, was a 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.

The secretary of state for foreign, commonwealth and development affairs, also known as the foreign secretary, is a secretary of state in the Government of the United Kingdom, with responsibility for the Foreign, Commonwealth and Development Office. The role is seen as one of the most senior ministers in the UK Government and is a Great Office of State. The incumbent is a member of the Cabinet of the United Kingdom and National Security Council, and reports directly to the prime minister.