End of Absolute Monarchy and Military rule

The Siamese revolution of 1932 was led by a group of young military officers and civil servants. The group held key figures, ministers who were of royal blood as hostages while the king, Rama VII, was at the summer palace in Hua Hin. The coup, usually called 'The Revolution of 1932', transformed the Government of Thailand from an absolute to a constitutional monarchy. The cabinet was presided by the prime minister. Military men always played a significant role in politics even before 1932. Already in 1912, during the Rama VI reign, young soldiers had been arrested who had plotted a coup urging a constitution and a change of the king's status.
King Rama VII, Prajadhipok initially accepted this change, granting the Constitution but later abdicated from his position due to conflicts with the government. The revolutionary government decided to install his ten-year-old nephew, Ananda Mahidol as the new monarch. Upon his abdication, King Prajadhipok said that the duty of a ruler was to reign for the good of the whole people, not for a selected few. Thai politics ran into turmoil as the revolutionary government plunged into factions, military, and intellectuals. A coup and a rebellion took place. Eventually, the military faction took control. The regime became evidently authoritarian under prime minister Luang Pibulsongkram, one of the members of the Revolutionary military wing.
The young King Ananda Mahidol (Rama VIII) died in 1946 under somewhat mysterious circumstances, the official explanation being that he shot himself by accident while cleaning his gun. He was succeeded by his brother Bhumibol Adulyadej, the longest-reigning king of Thailand, and very popular with the Thais. Although nominally a constitutional monarchy, Thailand was ruled by a series of military governments, most prominently led by Luang Phibunsongkhram and Sarit Dhanarajata, interspersed with brief periods of democracy.
In early January 1941, Thailand invaded French Indochina, beginning the French-Thai War. The Thais, better equipped and outnumbered by the French forces, easily reclaimed Laos. The French decisively won the naval Battle of Koh Chang.
The Japanese mediated the conflict, and a general armistice was declared on January 28. On May 9 a peace treaty was signed in Tokyo, with the French being coerced by the Japanese into relinquishing their hold on the disputed territories.
On December 8, 1941, a few hours after the attack on Pearl Harbor, Japan demanded the right to move troops across Thailand to the Malayan frontier. Japan invaded Thailand and engaged the Thai army for six to eight hours before Phibunsongkhram ordered an armistice. Shortly thereafter Japan was granted free passage, and on December 21, 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 (i.e., the Shan States of Burma, Malaya, Singapore, & part of Yunnan, plus Laos & Cambodia) Subsequently, Thailand undertook to 'assist' Japan in its war against the Allies. NOTE: Japan's distrust of Thailand extended to the point of rearming their 'ally' with controlled munitions, including the famous Siamese Mauser, which was manufactured in an unusual caliber. The Seri Thai (Free Thai Movement) was an underground resistance movement against Japan that was supported by the United States and operated freely, often with support from members of the Royal family (Prince Chula Chakrabongse) and members of the government.
After Japan's defeat in 1945, with the help of Seri Thai, Thailand was treated as a defeated country by the British and French, although American support mitigated the Allied terms. Thailand was not occupied by the Allies, but it was forced to return the territory it had regained to the British and the French. In the postwar period, Thailand had relations with the United States, which it saw as a protector from the communist revolutions in neighboring countries.
Communist guerillas existed in the country from the early '60s up to 1987, counting almost 12,000 full-time fighters at the peak of the movement, but never posed a serious threat to the state.
Recently, Thailand also has been an active member of the regional Association of Southeast Asian Nations (ASEAN), especially after democratic rule was restored in 1992.