The Suez Canal crisis ended the British Empire's pretension to superpower status

In 1956, Egyptian President Nasser nationalised the Suez Canal. Britain and France secretly colluded with Israel to invade Egypt and retake it. The operation was militarily successful but triggered fierce opposition from the United States, which threatened to sell its sterling reserves — a move that would have collapsed the pound. Britain was forced into a humiliating withdrawal within weeks. Prime Minister Eden resigned. Historians mark Suez as the moment Britain accepted it could no longer act as a great power independently of American approval — the psychological end of the British Empire.

Source: Kyle, K., Suez: Britain's End of Empire in the Middle East (2003)