Robert Stewart, 2nd Marquess of Londonderry

Robert Stewart, 2nd Marquess of Londonderry, KG, GCH, PC, PC (Ire) (18 June 1769 – 12 August 1822), known to history as Lord Castlereagh ( /ˈkɑːsəlreɪ/), was an Irish and British statesman. As British Foreign Secretary, from 1812 he was central to the management of the coalition that defeated Napoléon and was the principal British diplomat at the Congress of Vienna. Castlereagh was also leader of the British House of Commons in the Liverpool government from 1812 until his death by suicide in August 1822. Early in his career, as Chief Secretary for Ireland, he was involved in putting down the Irish Rebellion of 1798 and was instrumental in securing the passage of the controversial Irish Act of Union of 1800.

Robert Stewart acquired the courtesy title Viscount Castlereagh in 1796 when his father was created Earl of Londonderry in the Irish peerage. Upon his father's death in 1821, he succeeded as 2nd Marquess of Londonderry, a title to which his father had been raised in 1816. His younger half-brother, the soldier, politician and diplomat Charles Stewart (later Vane) succeeded him as 3rd Marquess of Londonderry.

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