Answer:
The formation of the United Kingdom of Great Britain and Northern Ireland has involved personal and political union across Great Britain and the wider British Isles. The United Kingdom is the most recent of a number of sovereign states that have been established in Great Britain at different periods in history, in different combinations and under a variety of polities. Historian Norman Davies has counted sixteen different states over the past 2,000 years.[1]

The evolution of the states of the British Isles. Those states evolved from the conquests and mergers of earlier states.
By the start of the 16th century, the number of states in Great Britain had been reduced to two: the Kingdom of England (which included Wales and controlled Ireland) and the Kingdom of Scotland. The once independent Principality of Wales fell under the control of English monarchs from the Statute of Rhuddlan in 1284. The Union of Crowns in 1603, the accidental consequence of a royal marriage one hundred years earlier, united the kingdoms in a personal union, though full political union in the form of the Kingdom of Great Britain required a Treaty of Union in 1706 and Acts of Union in 1707 (to ratify the Treaty).
The Act of Union 1800 united the Kingdom of Great Britain with the Kingdom of Ireland, which had been gradually brought under English control between 1541 and 1691, to form the United Kingdom of Great Britain and Ireland in 1801. Independence for the Irish Free State in 1922 followed the partition of the island of Ireland two years previously, with six of the nine counties of the province of Ulster remaining within the UK, which then changed to the current name in 1927 of the United Kingdom of Great Britain and Northern Ireland.
In the 20th century, the rise of Welsh and Scottish nationalism and resolution of the Troubles in Ireland resulted in the establishment of devolved parliaments or assemblies for Northern Ireland, Scotland and Wales.
Background
Formation of the Union
The "disuniting" of the United Kingdom
See also
Notes
