From March 1798 he served as acting chief secretary to Earl Camden, then Lord Lieutenant of Ireland, and, in November 1798, was formally appointed to that office by Camden's successor, Lord Cornwallis. While Lord Castlereagh was in office as chief secretary, two important events in Irish history took place: the 1798 rebellion and the union with Great Britain. He agreed with Lord Cornwallis that a policy of clemency was essential to end the disturbances.
The threat of a French invasion and the 1798 rebellion convinced him of the need for a parliamentary union with Britain. The passage of the Act of Union through the Dublin Parliament in June 1800 provided the first great demonstration of his ability as he single-handedly forced the measure in the Irish Commons against bitter Protestant
opposition. He believed that the union with Britain must be accompanied by the political emancipation of Roman Catholics.
When, in February 1801, Pitt failed to obtain George III's consent to emancipation, Cornwallis and Castlereagh at once sent in their resignations. Though out of office after May 1801, he continued to advise Henry Addington's ministry on Irish questions, and in July 1802 he was appointed president of the Board of Control responsible for Indian affairs.
His energy and intellectual powers gained him an immediate influence in the Cabinet and, after Pitt's return as prime minister, in July 1805 he also became secretary of state for war. His first important task, the dispatch of a British expeditionary force to Hanover, was, in December 1805, rendered ineffectual by Napoleon's victory at Austerlitz; but the move convinced Castlereagh of the strategic value of the British Army in continental warfare.
On Pitt's death in January 1806, he left office and became the chief opposition spokesman on foreign and military affairs. In 1807 he returned to the War Department in the Duke of Portland's ministry and showed his determination to engage in major warfare against a
continent now completely dominated by Napoleon. In 1808 Spain revolted against Napoleon and at once it was decided to send a major military expedition. Castlereagh was influential in securing the command for Sir Arthur Wellesley, the future Duke of Wellington.

In 1809 a British expedition sent by Castlereagh against the French naval base at Antwerpen was allowed to waste away of disease on the island of Walcheren. The disaster was in no way Castlereagh's fault, but it brought to a head the long-standing divisions and
intrigues in the Cabinet. Since March 1809, George Canning, the foreign secretary, had been pressing for a change of policy, and even before the Walcheren expedition he had secured a secret agreement to replace Castlereagh with Marquess Wellesley. When Castlereagh learned of the position in which Canning had placed him, he challenged him to
a duel that was fought on 21 September 1809. Canning was slightly wounded and both men resigned office. Castlereagh remained out of office for the next two and a half years.
In 1812 he rejoined the government as secretary for foreign affairs, and after Prime Minister Perceval's assassination he became leader of the House of Commons. British foreign policy then passed for a decade under unified control. His first task was to unite the shaky
and distrustful elements opposing Napol‚on. The British required the return of the Bourbon Monarchy in France and the separation of the Low Countries as a separate kingdom.
At the Congress of Vienna, he was able to play a commanding role and, by strengthening the weak Central European areas, prevented the aggrandizement of Russia. Castlereagh and Metternich dominated the inner negotiations, but it was Castlereagh who took the lead in resisting the territorial demands of Russia and Prussia. His aim was to make possible diplomacy by conference rather than interference by one country in the internal affairs of another.
His classic state paper of May 1820 emphasized the difference between the despotic states of eastern Europe and the constitutional structures of Britain and France, making it clear that the British government could act only on the expediency of any given issue and
within the limits of its parliamentary system. In 1821 the question of Greek independence and the fate of the Spanish colonies, affecting British political and commercial interests, made him decide to attend However, most of the diplomatic developments were kept from the British public, largely due to the personal nature of his diplomacy and his aloofness from public opinion. His apparent involvement with the eastern autocracies was disliked at home, while his role as spokesman for the government in the violent domestic politics of the He was involved with the unsuccessful introduction in 1820 of a bill to dissolve George IV's marriage with Queen Caroline. He was savagely attacked by such romantics as Lord Byron, Thomas Moore and Shelley. After the abortive Thistlewood plot to assassinate the
Cabinet in 1820, he always carried pistols in self-defence and, during the trial of Queen Caroline, was obliged to take up his residence in the Foreign Office for greater safety.

The burden imposed on him by the royal divorce affair of 1820, in addition to his duties at the Foreign Office and in the House of Commons, probably hastened his final collapse. In 1821 he showed signs of abnormal suspiciousness, which by 1822 became outright paranoia. It was due to the stress of work, but according to gossip, he was, or thought he was, being blackmailed on charges of homosexual acts and, on 12 August 1822, committed suicide, by cutting his throat with a pocket-knife, shortly before due to set out for Verona.
One of the most distinguished foreign secretaries in British history, he was equalled only by the Duke of Marlborough in the personal ascendancy that he gained as British representative in the European diplomacy of his time. He took a leading part in bringing
together the alliance of great powers that finally overthrew Napoleon and in deciding the form of the peace settlement of Vienna.
 

Source: Leo van de Pas