Western colonialism and imperialism suffered a set back in the wake of First World War due to rise of nationalism. As a result of the development of education and press, growth of democratic etc. there was a growth of political consciousness among the people of Asia and Africa and they began to demand self-government.
On the other hand, the imperialist powers could not keep them under control for long due to the presence of serious social and political problems in the colonies. Ultimately in the face of growing tide of nationalism and their weakened position, due to enormous losses suffered by them during the two world wars, the imperialist powers were obliged to withdraw from these colonies and grant independence to the people of Asia and Africa. It may be noted that the United Nations played a leading role in this process of decolonization.