Robert Stewart, 2nd Marquess of Londonderry, KG, GCH, PC (18 June 1769 in Dublin – 12 August 1822 at Loring Hall, Kent), generally known by his courtesy title of Viscount Castlereagh, which he held until 1821, was an Anglo-Irish politician who represented the United Kingdom at the Congress of Vienna. He was also intimately involved in securing the passage of the controversial Irish Act of Union.

Robert Stewart acquired the courtesy title Viscount Castlereagh in 1796 when his father was created Earl of Londonderry, and is generally known to history by that title, even though he was to become the Marquess of Londonderry before his death. The elder Robert Stewart was also known as The Viscount Castlereagh for about one year. He briefly became 2nd Marquess of Londonderry in the Peerage of Ireland on the death of his father in 1821.

Robert Stewart was born to the elder Robert Stewart, 1st Marquess of Londonderry, a landowner who was created an earl and subsequently a marquess by King George III, in Dublin in 1769. He was a great-nephew of Robert Cowan, a wealthy and successful Governor of Bombay for the British East India Company. Even before his father's election to the Irish House of Commons in 1771, this heritage put him squarely in the landed gentry class of Ulster Presbyterians whose ancestors first arrived in Ireland during the Plantation of Ulster. Stewart was born with a club foot which was successfully treated.

He received his secondary education at The Royal School, Armagh, and later attended St. John's College, Cambridge for one year. There, he learned Latin in addition to his native English. Later, he learned French, the language of his trade of diplomacy.

In 1790, Stewart was elected as a Member of Parliament for County Down and entered the Irish House of Commons as a Whig on a platform supporting electoral reform and Catholic emancipation. He was, however, enrolled in the militia shortly thereafter as an officer, a matter of course for a young aristocrat, and got to see very little of the Commons. In 1794, he won the English seat of Tregony on a similar platform. In 1795, he crossed the floor to join the Tories, but his initial principles of reform and emancipation continued to hold a place in his political thought.

In 1794, Stewart married Emily Hobart, a woman noted in contemporary accounts for her attractiveness and eccentricity. By all accounts, the two remained devoted to each other to the end, but they had no children. They did, however, care for the young Frederick Stewart, Stewart's nephew, while his father was serving in the army.

By 1797, he had risen to the post of Chief Secretary for Ireland. In this capacity, he played a key role in quashing the Irish Rebellion of 1798, offering clemency to commoners who had supported the rebellion, and focusing instead on pursuing rebel leaders. In 1800, he began lobbying in the Irish and British Parliaments for an official union between the two, convinced that it was the best way to soothe the long-standing sectarian divides in Ireland. With a thorough display of Machiavellian tactics including bribery, Castlereagh succeeded in steering the Irish Act of Union through both parliaments.

During his campaign for the Act of Union, he had promised that Irish Catholics would be allowed to sit in parliament, a move that was opposed by much of the British establishment, including George III. When it came to light in that the king had approached Henry Addington, an opponent of emancipation, about becoming Prime Minister to replace the pro-emancipation Pitt, both Castlereagh and Pitt resigned in protest.

After a few years, tensions between Tories supporting emancipation and those opposing it had relaxed, and Castlereagh returned to the cabinet as President of the Board of Control. In 1804, after much urging by Castlereagh, Pitt returned as Prime Minister, and Castlereagh was promoted to Secretary of State for War and the Colonies. After Pitt's death in 1806, Castlereagh resigned amidst the chaos of the Ministry of All the Talents. After that cabinet collapsed, Castlereagh again became Secretary of State for War and the Colonies in the Duke of Portland's administration in 1807.

In that role he became involved in disputes with Foreign Secretary George Canning over the failure of the Walcheren Expedition. Canning saw it as a diversion of troops from the Peninsular War based on a hopeless plan. Castlereagh had the support of Wellington, however, and evidence later surfaced that Canning himself had interfered with the plan, selecting the Earl of Chatham to command the expedition. The government became increasingly paralysed by disputes between the two men. Portland was in deteriorating health and gave no lead, until Canning threatened resignation unless Castlereagh was removed and replaced by Lord Wellesley. Portland secretly agreed to make this change when it would be possible.

Castlereagh discovered the deal in September of 1809 and demanded redress. He challenged Canning to a duel, which Canning accepted. Canning had never before fired a pistol. The duel was fought on 21 September 1809. Canning missed but Castlereagh wounded his opponent in the thigh. There was much outrage that two cabinet ministers had resorted to such a method. This forced both of their resignations from the government. Six months later, Canning published a full account of his actions in the affair, and many who had rallied to him became convinced Castlereagh had been betrayed by his cabinet colleague.

Three years later, in 1812, Castlereagh returned to the government, this time as Foreign Secretary, a role in which he served for the next ten years. He also became leader of the House of Commons in the wake of Spencer Perceval's assassination in 1812. In his role of Foreign Secretary he was instrumental in negotiating what has become known as a quadruple alliance between the United Kingdom, Austria, Russia and Prussia at Chaumont in March 1814, in the negotiation of the Treaty of Paris that brought peace with France, and at the Congress of Vienna.

At the Congress of Vienna, Castlereagh designed and proposed a form of collective and collaborative security for Europe, then called a Congress system. In the Congress system, the main signatory powers met periodically (every two years or so) and collectively managed European affairs. This system was successfully used to resolve the Polish-Saxon crisis at Vienna and the question of Greek independence at Laibach. The following ten years saw five European Congresses where disputes were resolved with a diminishing degree of effectiveness. Finally, by 1822, the whole system had collapsed because of the irreconcilable differences of opinion between the United Kingdom, Austria and Russia, and because of the lack of support for the Congress system in British public opinion. The Holy Alliance, which Castlereagh opposed, lingered for some time, however, and even had effects on the international stage as late as the Crimean war. The order created by the Congress of Vienna was also more successful than Congresses themselves, preventing major European land wars until the First World War a century later. Scholars and historians have seen the Congress system as a forerunner of the modern, arguably more effective collective security, international unity, and cooperative agreements of NATO, the EU, the League of Nations and the United Nations.

In the years 1812 to 1822, Castlereagh continued to competently manage Britain's foreign policy, generally pursuing a policy of continental engagement uncharacteristic of British foreign policy in the nineteenth century. Castlereagh was not known to be an effective public speaker and his diplomatic presentation style was at times abstruse. In Ireland he is always regarded, particularly by nationalists, as a traitor. He nonetheless developed a reputation for integrity, consistency and good will, which was perhaps unmatched by any diplomat of that era. His views on foreign policy were, unfortunately for him, ahead of his time.

Despite his many achievements, Castlereagh was extremely unpopular within the country as a result of his supposed reactionism abroad, and his association with the repressive measures of the Home Secretary, Lord Sidmouth. He attracted this last criticism because, as the Leader of the House of Commons, he was often called upon to defend his colleagues' policies in that most public of British forums. He had to defend the almost universally despised measures taken by Sidmouth and the others in order to remain in cabinet and continue his diplomatic work. For these reasons, Castlereagh is immortalised next to others in Lord Liverpool's Cabinet in Shelley's poem The Masque of Anarchy, a poem heavily critical of, and inspired by the Peterloo massacre:

:I met Murder on the way – :He had a face like Castlereagh – :Very smooth he looked, yet grim; :Seven bloodhounds followed him

:All were fat; and well they might :Be in admirable plight, :For one by one, and two by two, :He tossed them human hearts to chew :Which from his wide cloak he drew.

After the death of his father in 1821 he became Lord Londonderry. The next year, he began to suffer from a form of paranoia or a nervous breakdown, possibly as a result of an attack of gout and the stress of public criticism and the weak British position at the European Congresses. At the time, he said "My mind, is, as it were, gone." Londonderry returned to his country seat at Loring Hall in Water Lane, North Cray, Kent on the advice of his doctor. On 9 August 1822 he had an audience with King George IV in which he revealed to the King that he thought he was being blackmailed. He said, "I am accused of the same crime as the Bishop of Clogher." Percy Jocelyn, the Bishop of Clogher until that July, was prosecuted for homosexuality, and Castlereagh believed he was being blackmailed for the same reason. Whether this was true or a function of his paranoia is still unclear. The King is said to have advised Castlereagh to "consult a physician". On 12 August, Castlereagh committed suicide by cutting his throat with a letter opener.

An inquest concluded that the act had been committed whilst insane, avoiding the harsh strictures of the felo de se verdict that would have seen the suicide victim buried with a stake in his heart at a crossroads – an action that last occurred in 1823 before the law was amended in the same year. Some radicals, notably William Cobbett, construed this to be indicative of a "cover-up" within the government and a damning indictment of the elitism and privilege of the unreformed electoral system. His funeral on 20 August was greeted with jeering and insults along the processional route, although not to the level of unanimity projected in the radical press. Lord Londonderry was buried in the Abbey in the shadow of his mentor, William Pitt the Younger. A funeral monument was not erected until 1850 by his half-brother and successor, the 3rd Marquess of Londonderry.

Sometime after Castlereagh's death, Lord Byron wrote a sarcastic quip about his grave:

:Posterity will ne'er survey :A nobler grave than this: :Here lie the bones of Castlereagh: :Stop, traveller, and piss.

And yet, some of Castlereagh's political opponents were gracious in their epigrams. Henry Brougham, a Whig politician and later the Lord Chancellor, wrote:

:Put all their other men together in one scale, and poor Castlereagh in the other – single he plainly weighed them down... One can't help feeling a little for him, after being pitted against him for several years, pretty regularly. It is like losing a connection suddenly. Also he was a gentleman, and the only one amongst them.

A blue plaque is displayed at the entrance to Loring Hall, now a mental health facility and listed mansion, in commemoration of its most famous resident, who occupied the property from 1811.

Castlereagh's titles from birth to death: *Robert Stewart, Esquire (1769–1789) *The Honourable Robert Stewart (1789–1796) *The Viscount Castlereagh (1796–1798) *The Right Honourable The Viscount Castlereagh (1798–1821) *The Most Honourable The Marquess of Londonderry (1821–1822)

*Harold Nicolson, 'The Congress of Vienna' (1946) *H. Montgomery Hyde, 'The Strange Death of Lord Castlereagh' *John Derry, 'Castlereagh' (London 1976) *Wendy Hinde, 'Castlereagh' (London 1981) *Charles Webster, The Foreign Policy of Castlereagh * Robert Stewart, 'The Lightning Rod for Lord Liverpool's Government: Lord Castlereagh and HIs Critics, 1812 to 1822' (Unpublished M.A. Thesis, The University of Western Ontario, London, Ontario 1993)

*A letter to his Lordship, on the Peterloo massacre. *Private correspondence of the Viscount Castlereagh.