While maintaining separate parliaments, England and Scotland were ruled under one crown beginning in 1603, when James VI of Scotland succeeded his cousin Elizabeth I as James I of England. In the ensuing 100 years, strong religious and political differences divided the kingdoms. Finally, in 1707, England and Scotland were unified as Great Britain, sharing a single Parliament at Westminster.

Ireland's invasion by the Anglo-Normans in 1170 led to centuries of strife. Successive English kings sought to conquer Ireland. In the early 17th century, large-scale settlement of the north from Scotland and England began. After its defeat, Ireland was subjected, with varying degrees of success, to control and regulation by Britain.

The legislative union of Great Britain and Ireland was completed on January 1, 1801, under the name of the United Kingdom. However, armed struggle for independence continued sporadically into the 20th century. The Anglo-Irish Treaty of 1921 established the Irish Free State, which subsequently left the Commonwealth and became a republic after World War II. Six northern, predominantly Protestant, Irish counties have remained part of the United Kingdom.

The foundations of sea power were gradually laid to protect English trade and open up new routes. Defeat of the Spanish Armada in 1588 firmly established England as a major sea power. Thereafter, its interests outside Europe grew steadily. Attracted by the spice trade, English mercantile interests spread first to the Far East. In search of an alternate route to the Spice Islands, John Cabot reached the North American continent in 1498. Sir Walter Raleigh organized the first, short-lived colony in Virginia in 1584, and permanent English settlement began in 1607 at Jamestown, Virginia. During the next two centuries, Britain extended its influence abroad and consolidated its political development at home.

Great Britain's industrial revolution greatly strengthened its ability to oppose Napoleonic France. By the end of the Napoleonic Wars in 1815, the United Kingdom was the foremost European power, and its navy ruled the seas. Peace in Europe allowed the British to focus their interests on more remote parts of the world, and, during this period, the British Empire reached its zenith. British colonial expansion reached its height largely during the reign of Queen Victoria (1837-1901). Queen Victoria's reign witnessed the spread of British technology, commerce, language, and government throughout the British Empire, which, at its greatest extent, encompassed roughly one-fifth to one-quarter of the world's area and population. British colonies contributed to the United Kingdom's extraordinary economic growth and strengthened its voice in world affairs. Even as the United Kingdom extended its imperial reach overseas, it continued to develop and broaden its democratic institutions at home.

Britain's control over its empire loosened during the interwar period. Ireland, with the exception of six northern counties, gained independence from the United Kingdom in 1921. Nationalism became stronger in other parts of the empire, particularly in India and Egypt.

In 1926, the United Kingdom, completing a process begun a century earlier, granted Australia, Canada, and New Zealand complete autonomy within the empire. They became charter members of the British Commonwealth of Nations (now known as the Commonwealth), an informal but closely-knit association that succeeded the empire. Beginning with the independence of India and Pakistan in 1947, the remainder of the British Empire was almost completely dismantled. Today, most of Britain's former colonies belong to the Commonwealth, almost all of them as independent members. There are, however, 13 former British colonies--including Bermuda, Gibraltar, the Falkland Islands, and others--which have elected to continue their political links with London and are known as United Kingdom Overseas Territories.

Although often marked by economic and political nationalism, the Commonwealth offers the United Kingdom a voice in matters concerning many developing countries. In addition, the Commonwealth helps preserve many institutions deriving from British experience and models, such as parliamentary democracy, in those countries.