The United Kingdom of Great Britain and Ireland was a sovereign country in western Europe, the predecessor to the modern United Kingdom of Great Britain and Northern Ireland. It was established on 1 January 1801 by the Acts of Union 1800, which merged the parliaments of Great Britain and Ireland and combined the Crown of Ireland into the British Crown.
The United Kingdom financed the European coalition that defeated France in 1815 in the Napoleonic Wars, and with its unsurpassed Royal Navy and British Empire became the foremost world power for the next century. The Crimean War with Russia and the Boer wars were relatively small operations in a largely peaceful century. Rapid industrialisation that began in the decades prior to the state's formation continued up until the mid-19th century. The Great Irish Famine, exacerbated by government inaction in the mid-19th century, led to demographic collapse in much of Ireland and increased calls for Irish land reform.
The 19th century was an era of rapid economic modernization and growth of industry, trade, and finance, in which Britain largely dominated the world economy. Outward migration was heavy to the principal British overseas possessions and to the United States. The Empire was expanded into most parts of Africa and much of South Asia. The Colonial Office and India Office ruled through a small number of administrators who managed the units of the Empire locally, while democratic institutions began to develop. British India, by far the most important overseas possession, saw a short-lived revolt in 1857. In overseas policy, the central policy was free trade, which enabled British and Irish financiers and merchants to operate successfully in many otherwise independent countries, as in South America. London formed no permanent military alliances until the early 20th century, when it began to cooperate with Japan, France, and Russia, and moved closer to the United States.
The growing desire for Irish self-governance led to the Irish War of Independence after the First World War, which resulted in most of Ireland seceding from the Union and forming the Irish Free State in 1922. Northern Ireland remained part of the United Kingdom, and the state was consequently renamed the "United Kingdom of Great Britain and Northern Ireland".
A brief period of limited independence for Ireland came to an end following the Irish Rebellion of 1798, which occurred during the British war with revolutionary France. The British government's fear of an independent Ireland siding against them with the French resulted in the decision to unite the two countries. This was brought about by legislation in the parliaments of both kingdoms and came into effect on 1 January 1801. The Irish had been led to believe by the British that their loss of legislative independence would be compensated with Catholic Emancipation, that is, by the removal of civil disabilities placed upon Roman Catholics in both Great Britain and Ireland. However, King George III was bitterly opposed to any such Emancipation and succeeded in defeating his government's attempts to introduce it.