The military history of the United Kingdom covers the period from the birth of the United Kingdom of Great Britain in 1707, with the political union of England and Scotland, to the present day. From the 18th century and with the expansion of the British Empire its military became the largest and most powerful in the world but declined during the 20th century in the wake of two world wars, decolonisation and the rise of the United States and USSR as superpowers. The British Armed Forces encompasses the Royal Navy, the British Army, and the Royal Air Force.