The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a sovereign country in western Europe. Lying off the north-western coast of the European mainland, the United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state‍—‌the Republic of Ireland.Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to its east, the English Channel to its south and the Celtic Sea to its south-south-west, giving it the 12th-longest coastline in the world.

The United Kingdom has sovereignty over seventeen territories which do not form part of the United Kingdom itself: fourteen British Overseas Territories and three Crown dependencies.

The United Kingdom is a developed country and has the world's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity. The UK is considered to have a high-income economy and is categorised as very high in the Human Development Index, ranking 16th in the world. It was the world's first industrialised 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 recognised nuclear weapons state and is seventh in military expenditure in the world. The UK has been a permanent member of the United Nations Security Council since its first session in 1946. The UK is also a member of the Commonwealth of Nations, the Council of Europe, the G7 finance ministers, the G7 forum, the G20, NATO, the Organisation for Economic Co-operation and Development (OECD), and the World Trade Organization (WTO).

The armed forces of the United Kingdom—officially, Her Majesty's Armed Forces—consist of three professional service branches: the Royal Navy and Royal Marines (forming the Naval Service), the British Army and the Royal Air Force.The forces are managed by the Ministry of Defence and controlled by the Defence Council, chaired by the Secretary of State for Defence. The Commander-in-Chief is the British monarch, Elizabeth II, to whom members of the forces swear an oath of allegiance. The Armed Forces are charged with protecting the UK and its overseas territories, promoting the UK's global security interests and supporting international peacekeeping efforts. They are active and regular participants in NATO, including the Allied Rapid Reaction Corps, as well as the Five Power Defence Arrangements, RIMPAC and other worldwide coalition operations.

The British armed forces played a key role in establishing the British Empire as the dominant world power in the 18th, 19th and early 20th centuries. Throughout its unique history the British forces have seen action in a number of major wars, such as the Seven Years' War, the Napoleonic Wars, the Crimean War, the First World War and the Second World War—as well as many colonial conflicts. By emerging victorious from such conflicts, Britain has often been able to decisively influence world events. Since the end of the British Empire, the UK has nonetheless remained a major military power. Following the end of the Cold War, defence policy has a stated assumption that "the most demanding operations" will be undertaken as part of a coalition.