Robert Stewart, the son the second Marquis of Londonderry, was born on 18th June, 1769. After his education at St. John's College, Cambridge, Stewart toured Europe. At the age of twenty-one he obtained the seat of County Down, a pocket borough under the control of the Marquis of Downshire, in the Irish Parliament. It is claimed that the election cost the Marquis of Londonderry £60,000. In 1794 Stewart also obtained the Tregony seat in the English House of Commons.
During his election campaign in Ireland, Robert Stewart advocated parliamentary reform. This included extending the vote to Roman Catholic freeholders. However, soon after his election he enrolled in the Londonderry Militia. As a result of his military duties, Lieutenant-Colonel Stewart rarely attended Parliament during the next few years.
Robert Stewart entered Parliament as a Whig but in 1795 he switched his support to William Pitt and the Tories. Pitt granted Stewart the title Lord Castlereagh and in 1797 appointed him as his Irish chief secretary. This was a time of great turmoil in Ireland and in 1798 Castlereagh played an important role in crushing the Irish uprising.
Castlereagh and Pitt became convinced that the best way of dealing with the religious conflicts in Ireland was to unite the country with the rest of Britain under a single Parliament. The policy was unpopular with the borough proprietors and the members of the Irish Parliament who had spent large sums of money purchasing their seats. Castlereagh appealed to the Catholic majority and made it clear that after the Act of Union the government would grant them legal equality with the Protestant minority. After the government paid compensation to the borough proprietors and promising pensions, official posts and titles to members of the Irish Parliament, the Act of Union was passed in 1801.
King George III disagreed with Pitt and Castlereagh's policy of Catholic Emancipation. When Pitt discovered that the king had approached Henry Addington to become his prime minister, he resigned from office. Addington took office but Lord Castlereagh refused to serve under him.
In 1802 Castlereagh accepted Addington's offer to return to the cabinet. His initial responsibility was India but he soon became the leading figure in developing Britain's foreign policy. Henry Addington resigned from office in May 1804 and was replaced by William Pitt as prime minister. Castlereagh was now giving the post of Secretary for War.
Castlereagh left office in 1807 but five years later the new prime minister, Lord Liverpool, appointed Castlereagh as his foreign secretary. Castlereagh concentrated his efforts to defeat Napoleon in Europe. In 1814 Castlereagh represented Britain at the Congress of Vienna. The agreement reached at Vienna resulted in the reinforcement of hereditary rule and the suppression of liberal and nationalist sentiments in Europe
In 1815 British forces were victorious at the Battle of Waterloo. The abdication of Napoleon and the successful conclusion of the French Wars improved the public standing of Castlereagh and Lord Liverpool. It was hoped that with the end of the conflict in Europe, Lord Liverpool's government would be able to concentrate on introducing the social reforms that were much needed in Britain.
In 1817 Britain endured an economic recession. Unemployment, a bad harvest and high prices produced riots, demonstrations and a growth in the Hampden Club movement. As leader of the House of Commons, Castlereagh in November, 1817, introduced the bill for the suspension of Habeas Corpus.
The economic situation gradually improved and Lord Liverpool's government hoped that a reduction in taxation would prevent a revival of radicalism when the suspension of Habeas Corpus came to an end in 1818. This was not the case, and the summer of 1819 saw a series of large gatherings in favour of parliamentary reform, culminating in the massive public meeting at Manchester on 16th August 1819.
Lord Liverpool and his government made it clear that he fully supported the action of the magistrates and the Manchester and Salford Yeomanry. Radicals reacted by calling what happened in St. Peter's Fields, the Peterloo Massacre, therefore highlighting the fact that Liverpool's government was now willing to use the same tactics against the British people that it had used against Napoleon and the French Army.
Lord Liverpool's government decided to take action to prevent further large meetings demanding social reform. In November 1819, Parliament was assembled and Castlereagh introduced in the House of Commons the severe measures that became known as the Six Acts. Castlereagh, who was the government's spokesman for civil matters in the House of Commons, along with Lord Liverpool and Lord Sidmouth, the Home Secretary, took the blame for these repressive measures and the men were booed whenever they appeared in public.
Castlereagh found this loss of popularity very painful. He became depressed and his doctor suggested that he retired to his estate at North Cray Place in Kent. On 12th August, 1822, Lord Castlereagh cut his throat with a penknife in his dressing-room and died almost immediately.