Short answers, pulled from the story.
Japan invaded Thailand on the 8th of December 1941, less than four hours after the attack on Pearl Harbor. The invasion lasted around five hours before the Thai government arranged a ceasefire and acceded to Japanese demands for military passage.
Thailand declared war on both the United Kingdom and the United States on the 25th of January 1942. However, the Thai ambassador in Washington, Seni Pramoj, refused to deliver the declaration, so the United States never formally declared war on Thailand in return.
The Free Thai Movement was a pro-Allied resistance movement that operated inside Thailand and abroad during World War II. By the end of the war it numbered around 90,000 guerrillas, supported by government officials allied to regent Pridi Banomyong, and it worked with the United States Office of Strategic Services.
Thailand annexed territories in Burma, Malaya, Laos, and Cambodia during the war, including the Shan States (Saharat Thai Doem), parts of Malaya (Si Rat Malai), and Laotian and Cambodian provinces. All were returned when Thailand agreed to cede its wartime gains as the price of admission to the United Nations in 1946.
Phibun and his ideological spokesman Luang Wichitwathakan copied propaganda techniques from Adolf Hitler and Benito Mussolini, using the government's monopoly on radio broadcasting to build a cult of personality. He issued twelve Cultural Mandates between 1939 and 1942, abolished traditional Thai performing arts, required Thais to rise in cinemas when his portrait was shown, and called himself "The Leader."
Pridi Banomyong declared Phibun's 1942 war declaration unconstitutional, sparing Thailand a formal surrender. An Anglo-Thai Peace Treaty was signed on the 1st of January 1946 and an Australian-Thai Peace Treaty on the 3rd of April 1946. Thailand ceded all wartime territorial gains to join the United Nations and received a substantial package of U.S. aid.