Plaek Phibunsongkhram allied Thailand with Japan during World War II.
Japan invaded Thailand on 8 December 1941, shortly before the attack on Pearl Harbor. After brief resistance, Phibunsongkhram’s government agreed to a ceasefire and allowed Japanese forces to pass through Thailand to attack British-held Malaya and Burma.
On 21 December 1941, Thailand and Japan signed a military alliance. Phibun later declared war on Britain and the United States on 25 January 1942, although Thailand’s ambassador in Washington refused to deliver the declaration to the American government. This helped the United States avoid treating Thailand as a formal enemy after the war.
The alliance was politically complicated. Japan permitted Thailand to expand into territories in British Malaya, Burma, and French Indochina, while Thailand retained its own government and armed forces. At the same time, the Free Thai Movement operated inside and outside the country against Japanese influence and Phibun’s collaborationist regime. Phibun was removed from office in 1944, and Thailand ultimately avoided the harshest postwar penalties.