Three reasons.
  1. The major powers in Asia were weakened simultaneously due to civil wars (China), independence movements (India), Great Depression (Western Europe) or under attack from Germany (France & Netherlands).
  2. Japan promised
    freedom to various independence movements in Asia (in Burma, Malay and Philippines). Since the colonial powers were widely hated many independence movements were tempted to support Japan, especially in the period before their horrors well known. For instance, plenty of well-trained ex-British Indian soldiers were fighting alongside Japan & these officers knew South East Asia terrain well.
  1. Japan had the element of surprise. In a span of few hours, Japan attacked Malay peninsula, Pearl Harbor, Guam and Philippines. It took a while for US and Britain to realize what hit them and rebuild their totally whacked Pacific fleet.

Here is how it worked:
  1. Japan first took over China in 1937 that was embroiled in a major civil war between Mao's Communists and Chiang-Kai-Shek's Nationalists. The anarchy proved a great to drive away the nationalists.
  2. In 1940, Germany defeated France. France was forced to cede its colony of IndoChina - Vietnam, Laos and Cambodia to Japan.
  3. In the same year, Germany ran over Netherlands. This exposed the Dutch East Indies (Indonesia) to an easy takeover by Japan.
  4. In 1941, minutes before the Pearl Harbor attack, Japan attacked the British colony of Malaya. Already battling Germany in Europe, Britain could not defend Malaya.
  5. Few hours later, they attacked the US colony of Philippines in a surprise attack. US was not prepared for an attack on its forces as it was not even in the war and could not imagine someone attacking the world's most powerful nation.
  6. The next day, another American colony of Guam was attacked and taken over. Guam was poorly defended by the Americans.
  7. In 1942, Japan was helped by the Burmese Independence Army to takeover Burma.

Japan attacked at lightning speeds on European colonies whose home territories were already under siege by Germany. The local independence movements caused from a general hatred for colonialism made the job quite easy for Japan.

But for the Naga warriors in the Battle of Kohima that turned tide against Japan, there was little challenge to Japanese expansion. Soon US, Britain and Australia rebounded to produce a stinging attack on Japan's Pacific colonies.

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