The United Kingdom, officially the United Kingdom of Great Britain and Northern Ireland but more commonly known as the UK or Britain, is a sovereign country lying off the north-western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state—the Republic of Ireland. Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. The Irish Sea lies between Great Britain and Ireland. With an area of 242,500 square kilometers (93,600 sq mi), the United Kingdom is the 78th-largest sovereign state in the world. It is also the 22nd-most populous country, with an estimated 66.0 million inhabitants in 2017.
In 1603, the kingdoms of England, Scotland and Ireland were united in a personal union when James VI, King of Scots, inherited the crowns of England and Ireland and moved his court from Edinburgh to London; each country nevertheless remained a separate political entity and retained its separate political, legal, and religious institutions.
Though previous attempts at uniting the two kingdoms within Great Britain in 1606, 1667, and 1689 had proved unsuccessful, the attempt initiated in 1705 led to the Treaty of Union of 1706 being agreed and ratified by both parliaments
During the 18th century, Britain was involved in the Atlantic slave trade. British ships transported an estimated two million slaves from Africa to the West Indies. Parliament banned the trade in 1807, banned slavery in the British Empire in 1833, and Britain took a leading role in the movement to abolish slavery worldwide through the blockade of Africa and pressing other nations to end their trade with a series of treaties. The world's oldest international human rights organisation, Anti-Slavery International, was formed in London in 1839.
In the early 19th century, the British-led Industrial Revolution began to transform the country. Gradually political power shifted away from the old Tory and Whig landowning classes towards the new industrialists. An alliance of merchants and industrialists with the Whigs would lead to a new party, the Liberals, with an ideology of free trade and laissez-faire. In 1832 Parliament passed the Great Reform Act, which began the transfer of political power from the aristocracy to the middle classes. In the countryside, enclosure of the land was driving small farmers out. Towns and cities began to swell with a new urban working class. Few ordinary workers had the vote, and they created their own organizations in the form of trade unions.
After the defeat of France at the end of the Revolutionary and Napoleonic Wars (1792–1815), Great Britain emerged as the principal naval and imperial power of the 19th century (with London the largest city in the world from about 1830). Unchallenged at sea, British dominance was later described as Pax Britannica ("British Peace"), a period of relative peace among the Great Powers (1815–1914) during which the British Empire became the global hegemon and adopted the role of global policeman. By the time of the Great Exhibition of 1851, Britain was described as the "workshop of the world". The British Empire was expanded to include India, large parts of Africa and many other territories throughout the world. Alongside the formal control it exerted over its own colonies, British dominance of much of world trade meant that it effectively controlled the economies of many regions, such as Asia and Latin America. Domestically, political attitudes favored free trade and laissez-faire policies and a gradual widening of the voting franchise. During the century, the population increased at a dramatic rate, accompanied by rapid urbanization, causing significant social and economic stresses. To seek new markets and sources of raw materials, the Conservative Party under Disraeli launched a period of imperialist expansion in Egypt, South Africa, and elsewhere. Canada, Australia, and New Zealand became self-governing dominions. After the turn of the century, Britain's industrial dominance was challenged by Germany and the United States.
Social reform and home rule for Ireland were important domestic issues after 1900. The Labour Party emerged from an alliance of trade unions and small socialist groups in 1900, and suffragettes campaigned for women's right to vote before 1914.
Britain fought alongside France and Russia, against Germany and its allies in the World War (1914–1918). British armed forces were engaged across much of the British Empire and in several regions of Europe, particularly on the Western front. The high fatalities of trench warfare caused the loss of much of a generation of men, with lasting social effects in the nation and a great disruption in the social order.
After the war, Britain received over a number of former Ottoman territories. The British Empire reached its greatest extent, covering a fifth of the world's land surface and a quarter of its population. However, Britain had suffered 2.5 million casualties and finished the war with a huge national debt.