There exist many studies on the era of colonialism and Japanese occupation which belong to the most sensitive period of Thai modern history. They are mostly basal on either inside or outside viewpoint. The inside viewpoint explains that the Thais have actively coped with the threats of colonization and successfully led the modernization of the country. The outside viewpoint, on the contrary, interprets that Thailand, even though it preserved its dependence in reality stood under Western economic domination after concluding the Bowring treaty in 1855 with Great Britain. Seen from the inside viewpoint, the pro-Japanese policy of the Thai government under the premiership of Phibunsongkhram was a kind of recourse to the last resort which was aimed to save Thailand`s sovereignty. The outside viewpoint, by comparison, points to the then Thai leaden` tendency of nationalism and militarism and criticizes that the Thai government pursued a expansionist policy in Mainland Southeast Asia through the cooperation with Japan. In Thai society dominates the inside viewpoint. So pro-Western activities or pro-Japanese collaboration carried out privately or on the level of the government have hardly aroused negative reactions among the Thais. Even pro-Japanese collaborators including Phibunsongkhram wet all declared not guilty in the war crimes trial, and a critical evaluation of Thai history during the Pacific War, especially the relationship between Thailand and Japan, has been neglected.