Decolonization, process whereby the non-self-governing territories (including colonies, protectorates, and condominiums) of Western imperial powers gained independence. The term’s usage is normally confined to the post-1945 period, when the British Empire, French Empire, Dutch Empire, Spanish Empire, Portuguese Empire and others either voluntarily granted independence to their former subject dominions or yielded to national liberation movements. (The forcible eviction of the colonial power by another outside power, as with Germany after 1918 or the end of Japanese control over Korea in 1945, is not usually classed as decolonization.) The administrative and judicial elements of this process were linked to the 20th-century decline in the ability of the European nation states to project economic and military power on a global basis.
II General Causes and Characteristics
Three interconnected factors produced large-scale decolonization in the years after 1945. Opposition developed within the non-self-governing territories to the continuation of colonial rule. Some colonial powers introduced political and constitutional changes that aimed eventually to transfer power. Also power-political changes in the international system, linked especially to World War II and the developing Cold War, compelled disengagement from the colonies.
Organized opposition to colonial rule, often referred to as nationalist movements, emerged at different times and took different forms. Initially, political pressure for self-government or independence came from elite groups, but in India, the Indian National Congress had become a mass movement challenging British rule by 1918. In Africa, conversely, no anti-colonial party capable of appealing to broad sections of the population emerged until after World War II. The origins of such movements often lay in the social and economic changes taking place within the colonial territories, and in the desire to replace traditional sources of authority, who had often benefited from collaboration with colonial rulers.
Early political organizations demanding greater self-government and/or independence were given a boost by World War II. A war fought for freedom was interpreted by many educated Asians and Africans as a war that would lead to independence from colonial rule. In Asia, the Japanese conquest of the British, French, and Dutch territories destroyed the old myth of the invincibility and superiority of the white man. Many colonial subjects enlisted in the armies fighting against the evils of fascism and the racist ideas of National Socialism. They often served far from their native towns and villages and were exposed to new ideas and experiences, including Western ideas of freedom and democracy.
As colonialism was challenged, colonial rulers attempted to justify their roles........
http://uk.encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=781533275
And Several more answers:
http://www.britannica.com/eb/topic-155242/decolonization
http://www.britannia.com/history/euro/4/2_2.html
http://www.who.int/global_health_histories/seminars/nairobi_february2007.pdf
http://www.polarizationandconflict.org/Papers/garoupa.pdf