The Japanese invasion of Thailand (Thai: การบุกครองไทยของญี่ปุ่น, RTGS: Kan Buk Khrong Thai Khong Yipun; Japanese: 日本軍のタイ進駐, romanized: Nihongun no Tai shinchū) occurred on 8 December 1941. It was briefly fought between the Kingdom of Thailand and the Empire of Japan. Despite fierce fighting in southern Thailand, the fighting lasted only five hours before ending in a ceasefire. Thailand and Japan then formed an alliance making Thailand part of the Axis alliance until the end of World War II. It occurred roughly two hours before the Japanese attack on Pearl Harbor.
Background
Hakkō Ichiu
The origin of Japanese invasion of Thailand can be traced to the principle of hakkō ichiu as espoused by Tanaka Chigaku in the mid- to late-1800s. Tanaka interpreted the principle as meaning that imperial rule had been divinely ordained to expand until it united the entire world. While Tanaka saw this outcome as resulting from the Emperor's moral leadership, Japanese nationalists used it in terms of freeing Asia from colonizing powers and establishing Japan as the leading influence in Asia. The concept became expressed in the New Order in East Asia (東亜新秩序, Tōa Shin Chitsujo).
In 1940, the concept was expanded by Prime Minister Fumimaro Konoe, who sought to create the Greater East Asia Co-Prosperity Sphere, including Japan, Manchukuo, China, and parts of Southeast Asia. This would, according to imperial propaganda, establish a new international order seeking "co-prosperity" for Asian countries which would share prosperity and peace, free from Western colonialism and domination under the umbrella of a benevolent Japan.The 30 man Number 82 Section (also known as the Taiwan Army Research Unit or Doro Nawa Unit) Strike South planning was formed in 1939 or 1940 to bring this about. In its final planning stages, the unit was commanded by Colonel Yoshihide Hayashi.
Prelude
As part of conquering Southeast Asia, the Japanese military planned to invade Malaya and Burma. In order to do this, they needed to make use of Thai ports, railways, and airfields. They did not want conflict with the Thai military, as this would delay the invasion and significantly reduce the element of surprise. The Japanese plan was seen by the Nazi government of Germany as helpful in diverting the United Kingdom's military forces, and thus assisting Germany in its own conflict.
Thailand had a well-disciplined military, and after a series of border skirmishes in 1940 had invaded neighbouring French Indochina to recover provinces lost in the Franco-Siamese crisis of 1893. The Japanese, who wanted to use the Indo-Chinese ports and air-bases, acted as negotiators to bring about a settlement between the French and Thais on 31 January 1941. As part of the process, secret discussions were held with Thai Prime Minister Plaek Phibunsongkhram, in which the Japanese military sought free passage through Thailand. Phibun had responded positively, but his later actions showed he may have been very uncertain, as he had concluded the British–Thai Non-Aggression Pact on 12 June 1940. By February, the British were beginning to suspect the Japanese were planning to attack their possessions in Southeast Asia and were concerned Japan might set up bases in Thailand to that end.
The situation Phibun faced was that France had now been defeated by Germany, and Britain was heavily engaged in Europe; the United States had until then taken a neutral stance on both the European war and the Japanese war with China; and Japan was a superpower with a growing buildup of forces in French Indochina. Phibun may have decided he had little choice, as his own forces would have been unable to defeat the Japanese by themselves. Thailand's invasion of French Indochina in 1940 also made it difficult for the United States government to support Phibun.
Midway through 1941, Phibun sought British and American guarantees of effective support if Japan invaded Thailand. Neither the United Kingdom nor the United States could give them, although British Prime Minister Winston Churchill was in favour of a public warning to Japan that an invasion of the Southeast Asian kingdom would result in a British declaration of war. However, the United States was unwilling to agree to this, and Britain was not prepared to make it alone.
By August, the United Kingdom and the United States had placed sanctions against Japan. (For further information, see the Hull note and the McCollum memo.) The Japanese sought to have the sanctions lifted by promising not to encroach on Thailand and withdrawing their forces from Indochina, provided the United States withdrew its support for China. This was unacceptable to both Britain and the United States because of its impact on China.
Final days
In late November, the British became aware of a probable attack on Thailand by Japan because of the rapid buildup of Japanese troops in Indochina. On 1 December 1941, Prime Minister Hideki 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. 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.
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. 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.
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.
— Winston Churchill's message to Plaek Phibunsongkhram.
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.
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.
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.
Military forces
Thailand
Thailand had a well-trained military of 26,500 men, together with a reserve force which brought the army's numbers up to about 50,000.
The Royal Thai Air Force possessed some 270 aircraft, of which 150 were combat aircraft, many of them American. Japan had provided Thailand with 93 more modern aircraft in December 1940.
The Royal Thai Navy was poorly trained and equipped, and had lost a substantial number of vessels in its conflict with French Indochina. However, the Thai Navy had two operating submarines, HTMS Matchanu and HTMS Wirun, which were a cause of concern for the Japanese commanders.
The Royal Thai Army started to set up the new military units in the Kra Peninsula including:
- Chumphon
- the 38th Infantry Battalion stationed at Ban Na Nian, Tambon Wang Mai, Muang District of Chumphon (9 km from Provincial Hall).
- Nakhon Si Thammarat
- the 39th Infantry Battalion stationed at Tambon Pak Phoon, Muang District of Nakhon Si Thammarat.
- the 15th Artillery Battalion stationed at Tambon Pak Phoon, Muang District of Nakhon Si Thammarat.
- Headquarter of the Sixth Division at Tambon Pak Phoon, Muang District of Nakhon Si Thammarat.
- Trang
- the 40th Infantry Battalion.
- Songkla
- the 5th Infantry Battalion stationed at Tambon Khao Kho Hong, Hat Yai District of Songkla, transferred from Bang Sue to Hat Yai by military train on 18 February 1940, the first unit that moved to the south.
- the 41st Infantry Battalion stationed at Suan Tun, Tambon Khao Roob Chang, Muang District of Songkla.
- the 13th Artillery Battalion stationed at Suan Tun, Tambon Khao Roob Chang, Muang District of Songkla.
- Pattani
- the 42nd Infantry Battalion stationed at Tambon Bo Thong, Nong Jik District of Pattani.
Japan
Army units
Japan had units of its 15th Army under Lieutenant General Shōjirō Iida and 25th Army under Lieutenant General Tomoyuki Yamashita stationed in Indochina. Both armies had combat aircraft units. The 15th Army was tasked with the attack on Burma and the 25th with Malaya and Singapore. In order to attack Burma, the 15th Army needed to pass over the Bangkok plain, while the 25th Army needed to attack Malaya via the Kra Peninsula. The attack through Thailand on Malaya and Singapore was planned by Colonel Masanobu Tsuji while he was part of Unit 82. The Japanese had about 100,000 troops who needed to pass through Thailand.
Naval forces
Ships from IJN 2nd Fleet under Admiral Nobutake Kondō provided support and cover for the landings in Thailand and at Kota Baru in Malaya.
The known IJN ships participating, apart from those sent to Kota Baru, were:
- the cruiser IJN Kashii escorted seven transports carrying troops of the 143rd Regiment from Saigon
- Destroyers Asagiri, Amagiri, Sagiri, Yugiri, Shirakumo, and Shinonome supported the 25th Army landings in southern Thailand
- Escort ship IJN Shimushu escorted transports Zenyo Maru, Miike Maru and Toho Maru' to Nakorn Sri Thammarat, southern Thailand, with more of the 143rd Regiment
- Merchant seaplane carriers Kamikawa Maru, Sagara Maru
In total there were 18 transports involved, which included three landing troops at Kota Baru.
Japanese invasion
Japanese troops invaded Thailand from Indochina and with landings south of Bangkok and at various points along the Kra Peninsula several hours after Thailand had not responded to their ultimatum. While the government debated a response, Phibun could not be located and was unaware of the ultimatum until late morning.