The Twentieth Century would witness the extremes of British imperialism as it grew to its greatest extent after victory in the First World War to all but disappearing by the end of the century as nationalism and decolonisation swept it away in the aftermath of the Second World War and wrapped up into the dynamics of the Cold War. This was to be a century of volatility in the history of Empire. The colonies would at least prove to be a source of strength and aid when Britain became locked in mortal combat with totalitarian regimes bent on creating their own empires. But the cost of helping the mother country was often the demand for increased powers of self-government or even outright independence. It is no exaggeration to state that the Empire came to Britain's rescue in its various hours of need during the first half of the century, but that the ideal of empire also burnt itself out as wars to free and liberate peoples in Europe highlighted the contradictions inherent with imperial subjugation and control. The Empire found a purpose but in doing so discovered that it was no longer a suitable institution for the era it was entering. The twentieth century saw the effective end of Empire.

(Source: http://www.britishempire.co.uk/timeline/timeline.htm)