linguistic:paraphrasing, structural:reformatting
Constitutional monarchy, World War II and Cold War (1932–1975)

A bloodless revolution took place in 1932, in which Prajadhipok was forced to grant the country's first constitution, thereby ending centuries of feudal and absolute monarchy. The combined results of economic hardships brought on by the Great Depression, sharply falling rice prices, and a significant reduction in public spending caused discontent among aristocrats. In 1933, a counter-revolutionary rebellion occurred which aimed to reinstate absolute monarchy, but failed. Prajadhipok's conflict with the government eventually led to abdication. The government selected Ananda Mahidol, who was studying in Switzerland, to be the new king.

Later that decade, the army wing of Khana Ratsadon came to dominate Siamese politics. Plaek Phibunsongkhram who became premier in 1938, started political oppression and took an openly anti-royalist stance. His government adopted nationalism and Westernisation, anti-Chinese and anti-French policies.

In 1939, there was a decree changing the name of the country from "Siam" to "Thailand". In 1941, Thailand was in a brief conflict with Vichy France resulting in Thailand gaining some Lao and Cambodian territories.

On 8 December 1941, the Empire of Japan launched an invasion of Thailand, and fighting broke out shortly before Phibun ordered an armistice. Japan was granted free passage, and on 21 December Thailand and Japan signed a military alliance with a secret protocol, wherein the Japanese government agreed to help Thailand regain lost territories. The Thai government then declared war on the United States and the United Kingdom. The United Kingdom, whose colony Malaya was under immediate threat from Thai forces, responded in kind, but the United States refused to declare war and ignored Thailand's declaration. The Free Thai Movement was launched both in Thailand and abroad to oppose the government and Japanese occupation. After the war ended in 1945, Thailand signed formal agreements to end the state of war with the Allies.

In June 1946, young King Ananda was found dead under mysterious circumstances. His younger brother Bhumibol Adulyadej ascended to the throne. Thailand joined the Southeast Asia Treaty Organization (SEATO) to become an active ally of the United States in 1954. Field Marshal Sarit Thanarat launched a coup in 1957, which removed Khana Ratsadon from politics. His rule (premiership 1959–1963) was autocratic; he built his legitimacy around the god-like status of the monarch and by channelling the government's loyalty to the king. His government improved the country's infrastructure and education. After the United States joined the Vietnam War in 1961, there was a secret agreement wherein the U.S. promised to protect Thailand.

The period brought about increasing modernisation and Westernisation of Thai society. Rapid urbanisation occurred when the rural populace sought work in growing cities. Rural farmers gained class consciousness and were sympathetic to the Communist Party of Thailand. Economic development and education enabled the rise of a middle class in Bangkok and other cities. In October 1971, there was a large demonstration against the dictatorship of Thanom Kittikachorn (premiership 1963–1973), which led to civilian casualties. Bhumibol installed Sanya Dharmasakti (premiership 1973–1975) to replace him, marking the first time that the king had intervened in Thai politics directly since 1932. The aftermath of the event marked a short-lived parliamentary democracy, often called the "era when democracy blossomed" (ยุคประชาธิปไตยเบ่งบาน).