How did World War II impact decolonization?

Decolonization, or the disintegration of the European and American empires, began long before World War II. During the early 20th century, ideas about nationalism and freedom reached developing nations, in part because many of their citizens had lived or traveled to Europe. In addition, the Russian Revolution in 1917 unleashed ideas about the freedom of poor and subjugated people through communism, and these ideas also began to have a global reach. At the end of World War I, the League of Nations promoted the idea of eventual independence for European colonies. The developing world responded to these currents of thought.

In addition, developing nations were proving that they could defeat the western powers. For example, Japan's victory in the Russo-Japanese War of 1904-1905 proved that an Asian country could defeat an empire that was in part European. During World War II, England promised India that it could eventually be independent so that India would stay loyal to Britain (India became independent in 1948). Many Asian countries were conquered by Japan during World War II, and after the war, these countries, such as the Philippines, wanted freedom rather than to return to American or European rule. Europeans also found administering the colonies very costly, and they did not have money to do so after the expense of fighting World War II. While the Cold War slowed the process of decolonization to some degree, this process had already been put into motion. 

What aspect of World War II was most crucial to the decolonization process?

I would argue that the most important aspect of WWII, when it comes to decolonization, was the loss of power by the United Kingdom, France, and other European countries.  These countries lost their aura of power and they also lost their role as world leaders.  Both of these factors made it harder for them to keep their colonies.

In WWII, France, Belgium, and the Netherlands were all defeated by Germany.  Britain was defeated in Asia by Japan.  These countries had held their colonies, in part, by seeming to be powerful.  Their aura of power had made it so that their colonial subjects tended to accept that colonization was inevitable.  When these countries were defeated (especially when Britain was defeated by a country made up of non-white people), the aura of power eroded and the colonized people were less willing to accept their fate.

WWII also led to a change in world leadership.  France and Britain lost their status as great powers and the United States became the sole leader of the free world.  This helped bring about decolonization in at least two ways.  First, it weakened France and Britain, making it much harder for them to afford the expenditures required to hold their empires. Second, since the US was largely (not completely, as it supported French control over Vietnam) opposed to colonialism, it put pressure on France and Britain to eventually set their colonies free.

Thus, WWII helped bring about decolonization largely because it weakened European colonial powers, making it harder for them to maintain their empires.