The relationships among the countries of the United Kingdom have changed over time. Wales was annexed by the Kingdom of England under the Acts of Union of 1536 and 1543. A treaty between England and Scotland resulted in a unified Kingdom of Great Britain in 1707, which in 1801, merged with the Kingdom of Ireland to form the United Kingdom of Great Britain and Ireland. In 1922, five-sixths of Ireland seceded from the country, leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland. The UK has fourteen Overseas Territories. These are the remnants of the British Empire which, at its height in the 1920s, encompassed almost a quarter of the world's land mass and was the largest empire in history. British influence can be observed in the language, culture, and legal systems of many of its former colonies.
The United Kingdom is a developed country and has the world's sixth-largest economy by nominal GDP and tenth-largest by purchasing power parity. The country is considered to have a high-income economy and is categorized as very high in the Human Development Index, currently ranking 14th in the world. It was the world's first industrialized country and the world's foremost power during the 19th and early 20th centuries. The UK remains a great power with considerable economic, cultural, military, scientific, and political influence internationally. It is a recognized nuclear weapons state and its military expenditure ranks fifth or sixth in the world. The UK has been a permanent member of the United Nations Security Council since its first session in 1946. It has been a member state of the European Union (EU) and its predecessor, the European Economic Community (EEC), since 1973; it is also a member of the Commonwealth of Nations, the Council of Europe, the G7, the G8, the G20, NATO, the Organization for Economic Co-operation and Development (OECD), and the World Trade Organization (WTO).