During the Age of Discovery in the 15th and 16th centuries, Portugal and Spain pioneered European exploration of the globe, and in the process established large overseas empires. Envious of the great wealth these empires generated, England, France, and the Netherlands began to establish colonies and trade networks of their own in the Americas and Asia. A series of wars in the 17th and 18th centuries with the Netherlands and France left England and then, following union between England and Scotland in 1707, Great Britain, the dominant colonial power in North America. It then became the dominant power in the Indian subcontinent after the East India Company's conquest of Mughal Bengal at the Battle of Plassey in 1757.
The independence of the Thirteen Colonies in North America in 1783 after the American War of Independence caused Britain to lose some of its oldest and most populous colonies. British attention soon turned towards Asia, Africa, and the Pacific. After the defeat of France in the Revolutionary and Napoleonic Wars (1792–1815), Britain emerged as the principal naval and imperial power of the 19th century. Unchallenged at sea, British dominance was later described as Pax Britannica ("British Peace"), a period of relative peace in Europe and the world (1815–1914) during which the British Empire became the global hegemon and adopted the role of global policeman. In the early 19th century, the Industrial Revolution began to transform Britain; so that by the time of the Great Exhibition in 1851, the country was described as the "workshop of the world". The British Empire expanded to include most of India, large parts of Africa and many other territories throughout the world. Alongside the formal control that Britain exerted over its own colonies, its dominance of much of world trade meant that it effectively controlled the economies of many regions, such as Asia and Latin America.
In Britain, political attitudes favoured free trade and laissez-faire policies and a gradual widening of the voting franchise. During the 19th century, Britain's population increased at a dramatic rate, accompanied by rapid urbanisation, which caused significant social and economic stresses. To seek new markets and sources of raw materials, the Conservative Party under Benjamin Disraeli launched a period of imperialist expansion in Egypt, South Africa, and elsewhere. Canada, Australia, and New Zealand became self-governing dominions.
By the start of the 20th century, Germany and the United States had begun to challenge Britain's economic lead. Subsequent military and economic tensions between Britain and Germany were major causes of the First World War, during which Britain relied heavily upon its empire. The conflict placed enormous strain on the military, financial and manpower resources of Britain. Although the British Empire achieved its largest territorial extent immediately after World War I, Britain was no longer the world's pre-eminent industrial or military power. In the Second World War, Britain's colonies in Southeast Asia were occupied by Japan. Despite the final victory of Britain and its allies, the damage to British prestige helped to accelerate the decline of the empire. India, Britain's most valuable and populous possession, achieved independence as part of a larger decolonisation movement in which Britain granted independence to most territories of the empire. The transfer of Hong Kong to China in 1997 marked for many the end of the British Empire. Fourteen overseas territories remain under British sovereignty.
After independence, many former British colonies joined the Commonwealth of Nations, a free association of independent states. The United Kingdom is now one of 16 Commonwealth nations, a grouping known informally as the Commonwealth realms, that share a monarch, Queen Elizabeth II.