in Europe (green & grey)
The United Kingdom having financed the European coalition that defeated France during the Napoleonic Wars, developed a large Royal Navy that led the British Empire to become 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, which resulted in most of Ireland seceding from the Union and forming the Irish Free State in 1922. Northern Ireland remained part of the Union, and the state was renamed to the current "United Kingdom of Great Britain and Northern Ireland" in 1927. The modern-day United Kingdom is the same country as the one from this period—a direct continuation of what remained after the secession—not an entirely new successor state.
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.
During the War of the Second Coalition (1799–1801), Britain occupied most of the French and Dutch overseas possessions, the Netherlands having become a satellite state of France in 1796, but tropical diseases claimed the lives of over 40,000 troops. When the Treaty of Amiens ended the war, Britain agreed to return most of the territories it had seized. The peace settlement was in effect only a ceasefire, and Napoleon continued to provoke the British by attempting a trade embargo on the country and by occupying the city of Hanover, capital of the Electorate, a German-speaking duchy which was in a personal union with the United Kingdom. In May 1803, war was declared again. Napoleon's plans to invade Great Britain failed, chiefly due to the inferiority of his navy, and in 1805 a Royal Navy fleet led by Nelson decisively defeated the French and Spanish at Trafalgar, which was the last significant naval action of the Napoleonic Wars.