During World War II, the Empire of Japan demanded the right to move troops across Thailand to the Malayan frontier. The Japanese invasion of Thailand on 8 December 1941 occurred in co-ordination with attacks throughout Asia and engaged the Royal Thai Army for six to eight hours before Plaek Phibunsongkhram ordered an armistice. Shortly thereafter, Japan was granted free passage, and on 21 December 1941, Thailand and Japan signed a military alliance with a secret protocol, wherein Tokyo agreed to help Thailand regain territories lost to the British and French.[27]
Subsequently, Thailand declared war on the United States and the United Kingdom on 25 January 1942, and undertook to "assist" Japan in its war against the Allies, while at the same time maintaining an active anti-Japanese Free Thai Movement. Approximately 200,000 Asian labourers (mainly romusha) and 60,000 Allied prisoners of war (POWs) worked on the Burma Railway, which is commonly known as the "Death Railway".[27]
After the war, Thailand emerged as an ally of the United States. As with many of the developing nations during the Cold War, Thailand then went through decades of political instability characterised by a number of coups d'état, as one military regime replaced another, but eventually progressed towards a stable, prosperous democracy in the 1980s.