Britain financed the European coalition that defeated France in 1815 in the Napoleonic Wars. Britain, 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. A devastating 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.
It 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 main colonies and to the United States of America. The Empire was expanded into all parts of Africa and much of Asia. The Colonial Office and India Office ruled through a small number of administrators who supervised local elites. India, by far the most important possession, saw a short-lived revolt in 1857. In foreign policy, the central policy was free trade, which enabled its 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.
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.
In 1806, Napoleon issued the series of Berlin Decrees, which brought into effect the Continental System. This policy aimed to eliminate the threat from the British by closing French-controlled territory to foreign trade. The British Army remained a minimal threat to France; it maintained a standing strength of just 220,000 men at the height of the Napoleonic Wars, whereas France's armies exceeded a million men—in addition to the armies of numerous allies and several hundred thousand national guardsmen that Napoleon could draft into the French armies when they were needed. Although the Royal Navy effectively disrupted France's extra-continental trade—both by seizing and threatening French shipping and by seizing French colonial possessions—it could do nothing about France's trade with the major continental economies and posed little threat to French territory in Europe. France's population and agricultural capacity far outstripped that of the British Isles, but it was smaller in terms of industry, finance, mercantile marine and naval strength.