In December 1941, over the course of only a few days, the Thai government moved from a public stance of neutrality to a military alliance with Japan. Thailand’s alliance with Japan would ultimately define Thailand’s role in World War II in the Pacific Theater.
After allying itself with Japan, Thailand would go on to declare war against the British and Americans as well as assist the Japanese in supplying their troops through the completion of the infamous Thailand-Burma Railway. Thus, it is valuable to examine how this rapid shift in government policy took place in Thailand.
Prior to the outbreak of war in the Pacific, Thailand was a quasi-independent state in which the British and French exercised considerable influence. The area to the east of the Menam Chao Phraya Basin fell within the French sphere of influence while the area to the west of the river basin lay within the British sphere. However, in December 1938, Phibunsongkhram took power in Thailand as a military dictator. Phibunsongkhram, also known as Phibun, maintained friendly relations between Thailand and Japan. Relations with Japan had been increasing amiable since the early 1930s and when paired with Thailand’s new strongly nationalistic policies which were anti-Chinese at home and pro-Japanese abroad, relations between Thailand and European powers were increasingly strained. Thai-European relations continued to falter when in November 1940, Phibun ordered the invasion of French territories in western Laos and northwestern Cambodia that had formerly been under Thai control. This move on the part of Thailand was strongly supported by Japan.
