Number of present-day Southeast Asian countries under Western control before WWII

Of the 11 present-day countries in Southeast Asia, only Thailand was independent at the outbreak of the Second World War, while the other 10 were in the hands of Western powers. This was one of several sources of Japan's resentment for the West prior to the Second World War, as Western industrial growth had been fueled by resources taken from Asia for centuries. The German invasion of France and the Netherlands in May 1940 greatly weakened the Western position in Southeast Asia, and Japan sought to capitalize on the turmoil in Europe and annex the region into its own empire.

Japanese expansion

Japan's expansion into Southeast Asia began with its invasion of French Indochina in September 1940, which was used to create a blockade China, with whom Japan was also at war. However, the United States' Pacific fleet remained a major threat in the area, and the U.S. was growing wary of Japan's ambitions (but had not yet become hostile). Because of this, Japan launched a surprise attack on the American naval base at Pearl Harbor, Hawaii, on December 7, 1941, to try and hinder the American response to its planned aggression in the Pacific. The attack was a success, and Japanese forces then began attacking and moving into other areas of Southeast Asia later that day (December 8th, local time), and had taken virtually all of the 11 countries within six months. The Dutch East Indies (Indonesia) was one of the most important assets taken by the Japanese, due to its abundant resources, especially fossil fuels. Most Indonesians welcomed the Japanese as liberators, but animosity quickly grew as millions were then put to work as forced laborers across Japan's territories, and up to four million Indonesian civilians would die of famine, abuse, or forced labor by the war's end.

End of WWI and independence

The Japanese advance stalled in New Guinea in mid-1942, where the joint forces of Australia and New Zealand were able to gain a foothold on the island. The United States victory in the Battle of Midway also dealt a heavy blow to the Japanese Navy in the Pacific, proving to be a turning point of the Pacific War. In the years that followed, the Allies gradually pushed the Japanese northwards through Southeast Asia, before Japan's eventual defeat in 1945. The post-WWII years in Southeast Asia were defined by the push for independence and decolonization - between 1945 and 1957, nine of the ten colonized countries would gain their independence (Timor-Leste remained Portuguese until 1975, when it was then invaded and annexed by Indonesia from 1976-2002).