Why?

Thailand in World War II officially adopted a position of neutrality until the five hour-long Japanese invasion of Thailand on 8 December 1941, which led to an armistice and military alliance treaty between Thailand and the Japanese Empire in mid-December 1941. At the start of the Pacific War, the Japanese Empire pressured the Thai government to allow the passage of Japanese troops to invade British-held Malaya and Burma. The Thai government under Plaek Phibunsongkhram (known simply as Phibun) considered it profitable to co-operate with the Japanese war efforts, since Thailand saw Japan – who promised to help Thailand regain some of the Indochinese territories (in today's Laos, Cambodia, and Myanmar) which had been lost to France – as an ally against Western imperialism. Axis-aligned Thailand declared war on the United Kingdom and the United States and annexed territories in neighbouring countries, expanding to the north, south, and east, gaining a border with China near Kengtung.[1]

After becoming an ally of the Empire of Japan, Thailand retained control of its armed forces and internal affairs. The Japanese policy on Thailand differed from their relationship with the puppet state of Manchukuo. Japan intended bilateral relationships similar to those between Nazi Germany and Finland, Bulgaria, and Romania.[2] However, Thailand at that time was labelled by both the Japanese and the Allies as the "Italy of Asia".[why?] [3][4]

Meanwhile, the Thai government had split into two factions: the Phibun regime and the Free Thai Movement, a well-organised, pro-Allied resistance movement that eventually numbered around 90,000 Thai guerrillas,[5] supported by government officials allied to the regent Pridi Banomyong. The movement was active from 1942, resisting the Phibun regime and the Japanese.[6] The partisans provided espionage services to the Allies, performed some sabotage activities, and helped engineer Phibun's downfall in 1944. After the war, Thailand returned the annexed territories but received little punishment for its wartime role under Phibun.

Thailand suffered about 5,569 military dead during the war, almost entirely due to disease. Deaths in combat included 150 in the Shan States, 180 on December 8, 1941 (the day of both the brief Japanese invasion and the failed British assault on the Ledge), and 100 during the brief Franco-Thai War.[7][8]

https://en.wikipedia.org/wiki/Thailand_in_World_War_II

jap scared of transgender? hahaha

Didn't Japan travel through Thai to Malaysia and Thai allow them free passage?

In late November, the British become aware of a probable attack on Thailand by Japan because of the rapid buildup of Japanese troops in Indochina.[11] On 1 December 1941, Prime Minister Tojo of Japan stated that he was uncertain where Thailand stood regarding allowing Japanese troops free passage through its territory, but was hopeful a clash could be avoided.[12] Further negotiations took place between the Japanese diplomatic representative, Tamara, and Phibun on 2 December. Phibun was prepared to look the other way if Japan invaded the Kra Peninsula, but wanted them to avoid passing through the Bangkok Plain. After further discussions on 3 December, Phibun agreed to passage through Thailand, provided Thailand could regain the territories ceded in the Anglo-Siamese Treaty of 1909, as well as Burma's Shan State.[13]

On 2 December, the Japanese military issued the order "Climb Mount Niitaka", which set in motion the war in the Pacific. The main invasion fleet for Operation "E", the invasion of Malaya and Thailand, sailed from Sanya, Hainan Island, China on 4 December.[14] Further troops and ships joined the fleet from Cam Ranh Bay, Indochina. While the Japanese were preparing, the British and Americans were formulating their response to the Japanese troop buildup and the potential invasion of Thailand. Phibun, on the same day he reached an agreement with the Japanese, was advised by the British that Thailand was about to be invaded by the Japanese.[15]

There is a possibility of imminent Japanese invasion of your country. If you are attacked, defend yourselves. The preservation of the true independence and sovereignty of Thailand is a British interest, and we shall regard an attack on you as an attack upon ourselves.

— Prime Minister Winston Churchill's message to Field Marshal Phibun Songkhram.[16]

At noon on 6 December, one of three RAAF No 1 Squadron Lockheed Hudsons on a reconnaissance flight over the South China Sea, located three Japanese ships steaming west, and about 15 minutes later, sighted the IJN Southern Expeditionary Fleet convoy, consisting of a battleship, five cruisers, seven destroyers and 22 transports. One of the two merchant seaplane tenders with the convoy, the Kamikawa Maru, launched a Mitsubishi F1M "Pete" floatplane to intercept the Hudson, which eluded it by taking cover in the clouds. A few minutes later, a second Hudson also sighted the convoy.[17]

Air Chief Marshal Sir Robert Brooke-Popham was advised of the sightings at 14:00. He was not authorised to take any action against the convoy, as Britain was not at war with Japan, the Japanese intentions were still unclear, and no aggressive action had yet been taken against British or Thai territory. He put his forces in Malaya on full alert and ordered continued surveillance of the convoy.

On 7 December at 03:00, Vice-Admiral Jisaburō Ozawa ordered patrols in the area between the convoy and Malaya. The convoy was about 100 nautical miles (190 kilometres) from Kota Bharu. There was heavy rain and zero visibility. The Kamikawa Maru and Sagara Maru launched 11 F1M2's and six Aichi E13A's. About 20 nmi (37 km) west northwest of Panjang Island at 08:20, an E13A1 ZI-26 from the Kamikawa Maru, piloted by Ensign Ogata Eiichi, spotted a No. 205 Squadron RAF Consolidated PBY Catalina reconnaissance flying boat (W8417), piloted by Warrant Officer William E Webb. Ogata attacked the Catalina from the rear, damaging it and destroying its radio. Ogata shadowed the Catalina for 25 minutes until five Nakajima Ki-27 "Nate" fighters from the JAAF's 1st Sentai in Indo-China arrived and shot it down. Webb and his crew were the first casualties of the Pacific War. Unaware of this incident, the British took no action. Ogata would later be killed in the Battle of the Coral Sea.[18]

At 23:00 on 7 December, the Japanese presented the Thai government with an ultimatum to allow the Japanese military to enter Thailand. The Thais were given two hours to respond.[19]

https://en.wikipedia.org/wiki/Japanese_invasion_of_Thailand

Jap: Open up or we'll make you open up.

Thai: *spreads legs

Also every Thai: We weren't invaded.
You can't rape the willing.

Siam was smart, avoided having similar disaster happens on Burma and many kingdoms on Indian subcontinent which fell into British East India greedy hands

They also 'gained' back Northern Malay states that ceded under Anglo-Siam Treaty 1909. So in theory, Thailand were in cohort with Japan during WWII and they were never punished for that.

There nothing wrong for being neutral in WW2 while regained back lost territories as bonus