By 1921 the British Empire covered about a third of the world's surface and had a population of around 470-570 million people.
The heavy cost of the war effort during WWII and its aftermath, however, made it hard for Britain to maintain its vast Empire.
De-colonisation therefore became a regular event as more and more territories wanted their independence from the Empire.
Once Britain felt that a democratic government was available to transfer power to, many peaceful disengagements took place in the 1960s such as:
Almost all of the former British colonies joined the Commonwealth of Nations, which replaced the notion of an 'empire'.