Pearl Harbor was attacked at roughly the same time as Japan launched a full offensive all across Asia in order to secure oil and rubber, among other goods, for its war machine. By taking out the US Navy, the hope was to eliminate all organized opposition to Japanese supremacy in the Pacific. The Japanese Army rolled over everyone but Thailand, whom was forced to sign a ceasefire and join the Japanese in war against the Allies.

Phibun's decision to sign an armistice with Japan effectively ended Churchill's hopes of forging an alliance with Thailand. Phibun also granted Japan permission to use Thailand as a base of operations to invade Malaya. Within hours after the armistice came into effect, squadrons of Japanese aircraft had flown into Songkla airfield from Indochina, allowing them to carry out air raids on strategic bases in Malaya and Singapore from a short distance. At the time of the ceasefire, Britain and the United States regarded Thailand as Japanese-occupied territory.

On 14 December, Phibun signed a secret agreement with the Japanese committing Thai troops in the Malayan Campaign and Burma Campaign. An alliance between Thailand and Japan was formally signed on 21 December 1941. On 25 January 1942, the Thai government declared war on the United States and the United Kingdom. In response, all Thai assets in the United States were frozen by the federal government. While the Thai ambassador in London delivered the declaration of war to the British administration, Seni Pramoj, Thai ambassador to Washington, D.C., refused to do so, instead organizing the Free Thai Movement.