How did World War 2 lead to the end of colonial empires?

Several colonies suppported their motherlands on the condition that after the war, they would become fully independent (like Canada and several African countries). Other countries, especially in Asia, saw their once all-powerful colonial masters reduced to internment camp inmates by the Japanese which fatally damaged their image of superiority. Japan also had rallied local support in the countries they conquered by advertising their invasions as 'liberation' wars from colonial occupation. After their defeat they left a lot of military hardware for the new nationalist movements to use.

Another result of WW 2 was that it left countries like Britain and France economically broken and practically bankrupt. Hanging on to their former colonial empires would only have been feasible with the financial and military support of the US. But the US had made it a strict matter of policy not to support (and even actively discourage) any effort of re-establishing colonial empires.