The United Kingdom of Great Britain and Northern Ireland (United Kingdom; UK; Britain) is a country and sovereign state to the north-west of mainland Europe. It comprises the island of Great Britain, the north-east part of the island of Ireland and many small local islands. Northern Ireland is the only part of the UK with a land border, sharing it with the Republic of Ireland.Apart from this land border, the UK is surrounded by the Atlantic Ocean, the North Sea, the English Channel and the Irish Sea. It is linked to France by the Channel Tunnel. ************************** The United Kingdom is a constitutional monarchy comprising four constituent countries: England, Northern Ireland, Scotland and Wales. The British monarch is also monarch and head of state of fifteen other Commonwealth Realms including Canada, Australia, New Zealand and Jamaica. The Crown Dependencies of the Channel Islands and the Isle of Man, formally possessions of the Crown, are not part of the UK but form a federation with it. The UK has fourteen overseas territories, all remnants of the British Empire, which at its height encompassed almost a quarter of the world's land surface. ************************************ Britain was the world's foremost power during the 19th and early 20th century,but the economic cost of two world wars and the decline of its empire in the latter half of the 20th century diminished Britain's role and status in global affairs. The UK remains a major political, cultural, economic and nuclear military power with the second highest defense spending in the world; it holds a permanent seat on the United Nations Security Council, is a Great Power and is a member of the G8, NATO, the European Union and the Commonwealth of Nations.