Here is what I came up with.
World War Two greatly undermined the idea of Western Imperialism over any colony that they were in possession of.
The First World War had originally created the idea of an end to Imperialism, with the Ottoman, Russian, German and Austro-Hungarian Empires all collapsing from the wars. Both the World Wars had seen heavy involvement from the population of colonies across the world e.g. Australian, Canadian and Indian troops in the British army, by the end of the wars there was an overwhelming sense of giving them the independance that they had worked hard for.
Both internally and externally, there was a sense that the idea of 'Empire' was something out of date in the modern world. The two world wars had arisen from Imperialist desire for Empire, and it was becoming hard to convince the populace that something like the British Empire was a force for good Democracy etc.
