The United Kingdom of Great Britain and Ireland was a sovereign state in Northwestern Europe that comprised the entirety of the British Isles between 1801 and 1922. It was established by the Acts of Union 1800, which merged the Kingdom of Great Britain and the Kingdom of Ireland into a unified state. The establishment of the Irish Free State in 1922 led to the remainder later being renamed the United Kingdom of Great Britain and Northern Ireland in 1927.

The United Kingdom, having financed the European coalition that defeated France during the Napoleonic Wars, developed a large Royal Navy that enabled the British Empire to become the foremost world power for the next century. For nearly a century from the final defeat of Napoleon following the Battle of Waterloo to the outbreak of World War I, Britain was almost continuously at peace with Great Powers. The most notable exception was the Crimean War with the Russian Empire, in which actual hostilities were relatively limited. However, the United Kingdom did engage in extensive offensive military operations in Africa and Asia, such as the Opium Wars with the Qing dynasty, to extend its overseas territorial holdings and influence.

Beginning in earnest in the second half of the nineteenth century, the Imperial government granted increasing levels of autonomy to locally elected governments in colonies where white settlers had become demographically and/or politically dominant, with this process eventually resulting in Canada, Australia, New Zealand, Newfoundland and South Africa becoming self-governing dominions. Although these dominions remained part of the British Empire, in practice dominion governments were permitted to largely manage their own internal affairs without interference from London, which was primarily responsible only for foreign policy.

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 modernisation 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 British 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.

The British remained non-aligned until the early 20th century when the growing naval power of the German Empire increasingly came to be seen as an existential threat to the British Empire. In response, London began to cooperate with Japan, France and Russia, and moved closer to the United States. Although not formally allied with any of these powers, by 1914 British policy had all but committed to declaring war on Germany if the latter attacked France. This was realized in August 1914 when Germany invaded France via Belgium, whose neutrality had been guaranteed by London. The ensuing First World War eventually pitted the Allied and Associated Powers including the British Empire, France, Russia, Italy and the U.S. against the Central Powers of Germany, Austria-Hungary and the Ottoman Empire. The deadliest conflict in human history up to that point, the war ended in an Allied victory in November 1918 but inflicted a massive cost to British manpower, materiel and treasure.

Growing desire for Irish self-governance led to the Irish War of Independence almost immediately after the conclusion of World War I, which resulted in British recognition of the Irish Free State in 1922. Although the Free State was explicitly governed under dominion status and thus was not a fully independent polity, as a dominion it was no longer considered to be part of the United Kingdom and ceased to be represented in the Westminster Parliament. Six northeastern counties in Ireland, which since 1920 were being governed under a much more limited form of home rule, opted-out of joining the Free State and remained part of the Union under this limited form of self-government. In light of these changes, the British state was renamed the United Kingdom of Great Britain and Northern Ireland on 12 April 1927 with the Royal and Parliamentary Titles Act. The modern-day United Kingdom is the same state, that is to say a direct continuation of what remained after the Irish Free State's secession, as opposed to being an entirely new successor state.