When war broke out in Europe in September 1939, Thailand declared its neutrality, much to the distress of France and England. Both European nations had colonies surrounding Thailand and hoped Thailand would support the Allied effort and prevent Japanese encroachment on their Pacific territory. But Thailand began moving in the opposite direction, creating a "friendship" with Japan and adding to its school textbooks a futuristic map of Thailand with a "Greater Thailand" encroaching on Chinese territory.
Thailand's first real conflict with the Allies came after the fall of France to the Germans and the creation of the puppet government at Vichy. Thailand saw this as an opportunity to redraw the borders of French Indochina. The Vichy government refused to accommodate the Thais, so Thai troops crossed into French Indochina and battled French troops. Japan interceded in the conflict on the side of the Thais, and used its political alliance with Germany to force Vichy France to cede 21,000 square miles to Thailand.
On December 8, 1941, the Japanese made an amphibious landing on the coast of Thailand, part of the comprehensive sweep of South Pacific islands that followed the bombing raid at Pearl Harbor, Hawaii. The Japanese had assistance, though: Thailand's prime minister, Lang Pipul, collaborated with the Japanese, embracing the Axis power's war goal of usurping territory in China and ruling over the South Pacific. Pipul wanted to partake in the spoils; toward that end, he declared war on the United States and England. In October, he took dictatorial control of Thailand and became a loyal puppet of the Japanese.
Also on this Day: Jan 25, 1905: World's largest diamond found