Robert Stewart, better known as Viscount Castlereagh, entered the Irish Parliament in 1790. Initially sympathetic to some parliamentary reform, he became a defender of government authority during the revolutionary crisis of the 1790s. As chief secretary for Ireland from 1798, he helped direct the response to the rebellion of that year, whose suppression involved severe violence. Working with Lord Cornwallis, he then played a central role in securing the union of the Irish and British parliaments, effective in 1801.

The union was achieved through patronage, political bargaining, and promises of reform, including hopes of Catholic emancipation that were not fulfilled. Castlereagh supported admission of Catholics to political rights but resigned with William Pitt when George III opposed the measure. As president of the Board of Control he dealt with the East India Company, and as secretary of state for war he directed aspects of Britain's struggle against Napoleon.

Castlereagh supported operations in the Iberian Peninsula and Arthur Wellesley's command, but the failed Walcheren expedition of 1809 damaged the government. Conflict with Foreign Secretary George Canning culminated in a duel that year and resignation. He returned to major office as foreign secretary in 1812, also becoming leader of the House of Commons. His diplomacy sought to maintain allied cooperation despite differing interests among Austria, Russia, Prussia, and Britain.

The Treaty of Chaumont in 1814 committed the principal allies to continued resistance against Napoleon. At the Congress of Vienna, Castlereagh worked for a European balance that would restrain future French expansion without permanently destabilizing France. He supported territorial settlements and an alliance system through which the great powers could consult. He also pressed for condemnation of the Atlantic slave trade, although implementation remained uneven.

After the war Castlereagh defended the British government's restrictive domestic policies, including the Six Acts passed following the Peterloo killings in 1819. He opposed giving the European congress system a general license to intervene against revolutions and increasingly emphasized Britain's freedom of action. Suffering an acute mental health crisis, he died by suicide in 1822. His career linked the contested incorporation of Ireland, repression at home, and diplomacy that helped shape the European settlement after the Napoleonic Wars.