Robert Stewart acquired the courtesy title Viscount Castlereagh in 1796 when his father was created Earl of Londonderry. He did not properly acquire the title until his father died in 1821, at which point in time he also became the Marquess of Londonderry, a title his father had been raised to in 1816. He is generally known to history, however, not as the 2nd Marquess of Londonderry but as Viscount Castlereagh, the title he was known by most of his adult life.
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 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.
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, although Wellesley was neither complicit with nor even aware of the arrangement. Portland secretly agreed to make this change when it would be possible.
Castlereagh discovered the deal in September 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.
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. 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.
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.
In a retrospective and therefore necessarily speculative diagnosis, a thoughtful recent study has linked various instances of (at the time) little explained illness to syphilis, possibly contracted at Cambridge: here Stewart’s undergraduate studies were interrupted by a mysterious illness first apparent during the closing months of 1787, and which kept him away from Cambridge through the summer of 1788. Later, there were unexplained illnesses in 1801 and 1807, the first described by a contemporary as ‘brain fever’ which would be consistent with syphilitic meningitis. In addition to the events surrounding the suicide itself, towards the end of his life there are increasing reports of exceptionally powerful rages and sudden bouts of uncharacteristic forgetfulness.
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 savage quip about his grave:
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.