When the Japanese attacked Great Britain and the USA in December 1941, they planned to use independent Thailand as the main launching pad for their invasions of British Malaya and Burma. They expected cooperation from the ambitious and increasingly dictatorial Thai Premier, Field Marshal Phibun Songkhram, whom they had helped gain territory from French Indochina after a brief border war earlier in the year. After brief initial Thai resistance, Phibun agreed to free passage for the Japanese forces, then within days agreed to an alliance that made Thailand Japan's first true Asian ally. British and American residents were interned and their properties confiscated. In January 1942, Phibun declared war on Great Britain and the USA, in May he sent his troops into the British Shan States, and in 1943 he accepted territory in the Shan territories and four states in northern Malaya proffered by the Japanese to ensure his continued support. Thailand had become a critical supply base for Japanese operations in Burma.

When the tide of war turned against the Axis Powers, Thailand found itself in difficult straits. Unless a way could be found to escape the embrace of Japan, Thailand would go down in flames, too; perhaps even lose its treasured independence. Phibun's chief civilian political rival, Pridi Phanomyong, sought to salvage the nation's position and gain the political upper hand by secretly seeking Allied support for a government-in-exile and an anti-Japanese underground.