Castlereagh was born Robert Stewart on 17 June 1769, the eldest surviving son of Robert Stewart, the first Marquess of Londonderry, and his first wife, Lady Sarah Frances, the second daughter of Francis Seymour Conway, Marquess of Hertford.
He was educated at Armagh in Ireland, and then at St. John’s College, Cambridge. He visited Paris, Geneva, Rome, and Vienna in 1788 and 1789, and on returning to Ireland in 1790 he contested and won one of the County Down seats in the Irish Parliament— but at a cost of £60,000, which nearly ruined his family. At the outbreak of the French wars in 1793 he became lieutenant colonel of the Londonderry militia.
He served as M.P for Tregony (1794—1796) in the English Parliament and as M.P. for Orford, Suffolk (1796-1797). In 1797, he was appointed to the chief secretaryship of Ireland. In this capacity he ordered the arrest of the United Irishmen who had rebelled against English rule in 1798 with the help of French troops. Shordy afterward he was involved in pressing for the Act of Union between Ireland and England and also for a degree of Catholic emancipation. The Act of Union was passed in 1800; but the king’s refusal to make any significant concessions to the Irish Catholics led Castlereagh to resign in 1801, following the example of William Pitt, the Younger, who had resigned as prime minister over the same issue. Casdereagh held no official position in Henry Addingtons ministry but was given the responsibility of conducting through Parliament two Irish measures—the Suppression of Rebellion Act and the Suspension of Habeas Corpus, the right of trial with a jury.
After the Act of Union of 1800, Castlereagh sat in the House of Commons for County Down until 1805, when he was defeated in the parliamentary election. He was elected for Boroughbridge, in Yorkshire, in January 1806. He was subsequently elected as M.P. for Plympton Earl in Devon in November 1806 and for County Down again in 1812, 1818, and 1820. On succeeding to the Irish peerage, he was elected for Orford in April 1821. This almost unbroken period of tenure in the English Parliament projected him forward in a succession of governments, particularly on foreign policy.
Encouraged by William Pitt, Castlereagh joined Addington’s administration as president of the (East India) Board of Control in 1802, which gave him a seat in the cabinet. In cabinet he spoke increasingly on foreign policy, almost acting as Pitt’s mouthpiece. After Pitt formed a new government in 1804, Castlereagh was given the additional post of secretary of war in 1805. On Pitt’s death he lost these responsibilities; but in 1807 he again became secretary of war, this time in the Duke of Portland’s ministry. In this post he supported Arthur Wellesley’s military campaigns in Spain and Portugal, during the Peninsular War.
Castlereagh returned to office as foreign secretary on 28 February 1812, in the latter days of Spencer Percevals ministry, and remained in that post under Lord Liverpool until his death in 1822.
Castlereagh became Lord Londonderry after the death of his father on 11 April 1821. By this point the enormous work burden and a number of health problems, particularly gout, were beginning to take their toll. It seems that his mind also was going: on 12 August 1822 he committed suicide by cutting his throat with a penknife; an inquest found him to have been of unsound mind. He was subsequently buried in Westminster Abbey, on 20 August 1822.
He mounted his own ill- fated expedition of40,000 troops and naval personnel to capture the island of Walcheren, off Holland, in an attempt to get France to divert its troops from other theaters of war. The expedition was mounted in July 1809 but had failed by September. As a result there was cabinet pressure for him to move from the post of secretary of war to that of Lord President of the Council. George Canning, the foreign secretary, was behind the attempt to remove Castlereagh; and Spencer Perceval, the new prime minister, showed Castlereagh the letters Canning had written to him on the subject. An incensed Castlereagh challenged Canning to a duel on Putney Heath on 21 September 1809. Canning was slightly wounded in the thigh, and both men were forced to resign from the government.
Initially, his main responsibility as a foreign secretary was to secure the alliance against Napoleon, dealing with the rival interests of Austria, Russia, Prussia, and other allied powers involved in the conflict. When the Congress of Vienna was called in September 1814, he and his brother Sir Charles, then Lord Stewart, represented Britain. Casdereagh’s main concerns were to limit France to its prerevolutionary borders, to restore the Bourbons to the throne, and to restrict the growing power of Russia—for which purpose he sought the creation of a Polish state and of the German confederation. In the end he had to compromise. In the case of Poland, for instance, instead of an independent Polish state to act as a buffer, the Prussians were given a defensible border against Russia; the Russian claims to Austrian Poland were reduced; and the tsar maintained control of Warsaw and its province. The negotiations were, however, interrupted by Napoleons campaign of 1815, and the Treaty of Vienna was not signed until June 1816, by which time Napoleon had been subdued and imprisoned on the island of St. Helena.
Castlereagh was then at the peak of his influence; but later his popularity declined to the point of expiry. This decline was partly because he was the government spokesman on home policy in the House of Commons, while Lord Sidmouth, the home secretary, was in the House of Lords. Among the many oppressive social and political measures he introduced to control the radical agitation of the time was the ending of trial by jury through the suspension of the Habeas Corpus Act in 1817. Castlereagh also introduced into the House of Commons the severe Six Acts in the wake of the Peterloo “Massacre” of 1819 to restrict the holding of reform meetings and give magistrates additional power to search for arms; and it was his head, as well as Sidmouth’s, that the Cato Street conspirators proposed to carry on pikes through the streets of London if their uprising was successful. As for as foreign policy was concerned, Castlereagh’s main concern was to pursue a policy of nonintervention and conciliation. For instance, he would not allow Britain to become involved in the revolutions that occurred in Spain, Portugal, and Piedmont in 1820 and 1821.
By any standards, Castlereagh was one of the greatest of British politicians. He helped defeat the Irish uprising of 1798, organized the Union of England and Ireland in 1800, played a major part in the defeat of Napoleon, and helped ensure the balance of power in Europe through the Treaty of Vienna.
Stewart generally voted with the Tories when he was able to attend sessions of the Irish Parliament, and was firm in his belief in the necessity of a parliamentary union between England and Ireland as well as of the removal of restrictions against the Catholics.
On 9 June 1794 he married Lady Emily Anne, the youngest daughter and coheiress of John Hobart, the second earl of Buckinghamshire.
With his fathers elevation to an earldom in 1796, he became Lord Casdereagh.
