World War II was, as the name suggests, a global combat, led by two Military alliances: The Allies (The USA, Russia, the UK and China) and The Axis (Germany, Italy and Japan). It took place from 1939 to 1945. Deaths in this period are estimated between 50 to 80 million.
From 1945 to 1952 over 30 nations achieved independence, mainly from Asia and Africa. There was huge spending in colonies that were <em>poor and underdeveloped</em>, and after the war, there was a lot of reconstruction and recovery to be done by the countries, so not only was it an ideological but also financial break from imperial countries. If you take India, for example, Britain would spend over $1 Billion per year in India, money that could be spent in Britain's recovery from the war instead of a colony. So colonies were a great source of resources but also a big financial responsibility.
Summarizing, after the end of the war, imperial countries had to focus their spending in their own territory to recover from the war, which opened the road of decolonization. Not all processes of independence were peaceful. Some countries had to go through revolutions, as others needed only to sign agreements. However, World War II really opened the way for decolonization.
