The 'Battle of Prachuap Khiri Khan' was one of the six engagements between Japanese and Thai forces during the first stage of the Japanese invasion and occupation of Thailand (8/9 December 1941).

The battle was fought at the airfield of Prachuap Khiri Khan on the western coast of the Gulf of Thailand, and more specifically on the eastern coast of the isthmus of Kra that formed the junction of Thailand and Malaya. The Japanese intended to use Thailand as a base for their 'B' (iii) and 'E' (i) invasions of the British possessions in Burma and Malaya, and their forces attacked Thailand without provocation or warning.

At about 03.00 on 8 December 1941, the 2/143^rd Regiment of Lieutenant General Hiroshi Takeushi’s Japanese 55^th Division, under the command of Major Kisoyoshi Utsunomiya, began to land troops at Prachuap Khiri Khan. When informed of the invasion, Wing Commander Mom Luang Prawat Chumsai of the 5^th Kong Bin Noi (squadron) issued an immediate order for Thai resistance.

The units on the airfield were equipped with six heavy and two light machine guns, which they immediately turned on the Japanese troops trying to surround the airfield. The small garrison of pilots and ground crew was reinforced by members of the local police and the Yuwachon Thaharn (a quasi-military teenage auxiliary) who had managed to escape from Prachuap Khiri Khan town after the Japanese captured the telegraph office and the police station.

Despite the fact that the Japanese occupied part of the airfield, the 5^th Kong Bin Noi’s pilots attempted to take off at sunrise to bomb and strafe the advancing Japanese.

Chief Warrant Officer Prom Chuwong was first to take off in a Curtiss Hawk III single-engine biplane fighter, but he was soon killed as Japanese ground fire shot down his aeroplane. The Japanese shot down two more Hawk fighters as they took off, killing both pilots, and wounded a third pilot as he brought his Hawk onto the runway. Only one other pilot managed to get airborne. Flying Officer Man Prasongdi took off in a Hawk III armed with four 110-lb (50-kg) bombs and attempted to attack the Japanese transport vessels in Ao Manao harbour, but he could not locate his targets as a result of heavy fog and rain.

By 08.00 most of the airfield’s northern hangars were in Japanese hands. The Thais smashed the instruments of the isolated airfield control tower and set fire to it as the runways were abandoned. A new perimeter was established and the withdrawing airmen were covered by a machine gun sited on the clubhouse’s tennis court. The machine gun kept firing throughout the morning and into the afternoon.

Pilot Officer Somsri Suchrittham and his men, now numbering about 30 other ranks, were forced to withdraw when their northern flank was threatened by the abandonment of the runways.

Having secured their beach-head, the Japanese moved forward to occupy what was left of the hangars and runways, and reinforcements, including artillery and 10 tanks, were landed from the transports.

The families of the airmen took refuge in guest houses on Mt Lom Muak. The evacuation of the living quarters was supervised by Pilot Officer Phol Thongpricha.

Another defensive position was established by defenders, who divided themselves into three groups: the first was stationed by the guest houses on Prachuap Bay and fired on anything coming up the road from the guard house; the second, under the command of Wing Commander Prawat, placed itself in the area around the command and administrative buildings' and the third occupied houses facing Manao Bay. The latter two groups fired on the approaches from the hangars and the runways.

Fighting continued late into the evening, albeit at steadily lessening intensity. The machine gun at the tennis court held back the Japanese, while a light machine gun was held in reserve and moved to plug gaps in the perimeter. Rumours that men of the Royal Thai navy were fighting their way through to relieve the airmen kept up Thai hopes through the night, but ammunition was running low, and at one point the airmen fired blank rounds at the Japanese.

During the morning of the following day, the exhausted Thais received a telegram from the ministry of the interior, brought in by a postman during a lull in the battle. This missive ordered the defenders to cease fighting as an armistice had been arranged. The Thai defenders suspected this was a trick by the Japanese, however, and therefore continued their resistance. The invading force now mounted assaults with renewed vigour, and the defenders were slowly pushed back. About this time, the lone machine gun on the tennis court was taken out, both gunners being severely wounded.

By 10.00, with the Japanese closing in, Prawat ordered the command building to be burned, along with all military documents. As flames engulfed the building, Flying Officer Prayad Kanchonwiroj, the senior medical officer, ordered the hospital building evacuated and set on fire. Prawat ordered all officers to save a bullet for themselves and said those who wished it were free to try to break out on their own. The others, including the wounded, were to fall back on Mt Lom Muak.

At 12.00, a civilian car flying a small white flag arrived. It contained a number of Thai government officials, including the provincial undersecretary, Jarunphan Isarangun na Ayutthaya, who handed Prawat a direct order from the prime minister, Field Marshal Phibunsongkhram, ordering an immediate end to the resistance. The fighting ended at 12.35 on 9 December.

The Thais suffered the loss of 42 men killed and 27 wounded, these casualties including airmen, police and civilians. Among the dead was Prawat’s pregnant wife, who was struck by a stray bullet. Japanese sources stated that the Japanese had suffered the loss of 115 men killed, but Thai estimates claimed that the Japanese had lost 217 men killed and more than 300 wounded.

The Japanese invasion of Thailand, it is worth noting, began on 8 December and was a brief campaign fought between Thailand and 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, effectively making Thailand part of the Axis alliance until the end of World War II.

The origins of the Japanese invasion of Thailand can be traced to the principle of hakko ichiu as promulgated by Tanaka Chigaku in the mid- to late 19^th century. Tanaka interpreted the principle as meaning that the expansion of Japanese imperial rule had been divinely ordained to unite 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 colonising powers and establishing Japan as the leading influence in Asia. The concept became expressed in the New Order in East Asia. 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 South-East Asia. According to imperial propaganda, this would establish a new international order seeking 'co-prosperity' for Asian countries which would share prosperity and peace, and be free of Western colonialism and domination under the umbrella of a benevolent Japan. The 30-man No. 82 Section, which was also known as the Taiwan Army Research Unit Strike South was formed in 1939 or 1940 to bring this about. In its final planning stages, the unit was commanded by Colonel Yoshihide Hayashi.

As part of the conquest of South-East 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 wish conflict with the Thai military, as this would delay the primary invasions and significantly reduce the element of surprise. The Japanese plan was seen by the German government as helpful in diverting the UK’s military forces and thus assisting Germany in its own conflict.

Thailand had a well organised military, and after a series of border skirmishes in 1940 had invaded neighbouring Vichy French Indo-China to recover provinces lost in the Franco-Siamese War of 1893. Wishing to make use of Indo-Chinese ports and air bases, the Japanese acted as negotiators to bring about a settlement between the Vichy French and Thais on 31 January 1941. As part of the process, secret discussions were held with the 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 South-East Asia and were concerned Japan might establish bases in Thailand to further that end.

The situation faced by Phibun was that France had now been defeated by Germany, and the UK was heavily engaged in Europe; the USA had up to this time taken a neutral stance on both the European war and the Japanese war with China; and Japan was a superpower with a growing build-up of forces in Vichy French Indo-China. Phibun could have decided that he had little choice, as his own forces would have been unable to defeat the Japanese by themselves. Thailand’s invasion of Vichy French Indo-China in 1940 also made it difficult for the US government to support Phibun.

Mid-way through 1941, Phibun sought British and US guarantees of effective support in the event that Japan invaded Thailand. Neither the UK nor the USA could give such guarantees, although the British prime minister, Winston Churchill, favoured the issuance of a public warning to Japan that an invasion of the South-East Asian kingdom would result in a British declaration of war. However, the USA was unwilling to agree to this, and the UK was not prepared to proceed alone.

By August, the UK and the USA had imposed sanctions on Japan. The Japanese attempted to have the sanctions lifted by promising not to encroach on Thailand and withdrawing their forces from Indo-China, provided the USA withdrew its support for China. This was unacceptable to both the UK and the USA because of its impact on China.

Late in November, as a result of the rapid build-up of Japanese troops in Indo-China, the British became aware of a probable attack on Thailand by Japan. On 1 December 1941, the Japanese prime minister, General Hideki Tojo, stated that he was uncertain where Thailand stood regarding the possibility of allowing Japanese troops free passage through its territory, but was hopeful a clash could be avoided. More negotiations took place between Phibun and the Japanese diplomatic representative, Tamara, on 2 December. Phibun was prepared to look the other way if Japan landed troops on the isthmus of Kra, but wanted the Japanese to avoid passing through the Bangkok plain. After further discussions on 3 December, Phibun agreed to passage through Thailand, provided the latter 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 the 'E' (i) invasion of Malaya and Thailand, departed by sea from Sanya on the southern Chinese island of Hainan on 4 December. More troops and ships joined the fleet from Cam Ranh Bay in Vichy French Indo-China. While the Japanese were preparing, the British and Americans were formulating their response to the Japanese troop build-up and the possible invasion of Thailand. On the same day that he reached an agreement with the Japanese, Phibun 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.'

At 12.00 on 6 December, one of three Royal Australian Air Force No.1 Squadron Lockheed Hudson twin-engined aircraft on a reconnaissance flight over the South China Sea located three Japanese ships steaming west, and about 15 minutes later, sighted Vice Admiral Jisaburo Ozawa’s Southern Expeditionary Fleet convoy, comprising one battleship, five cruisers, seven destroyers and 22 transport ships. One of the two merchant seaplane tenders with the convoy, the Kamikawa Maru, launched a Mitsubishi F1M 'Pete' single-engined 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, the British commander-in-chief of the Far East Command, was advised of the sightings at 14.00. He was not authorised to take any action against the convoy, as the UK 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.

At 03.00 on 7 December, Ozawa ordered patrols in the area between the convoy and Malaya. The convoy was about 115 miles (185 km) from Kota Bharu on Malaya’s north-eastern coast. There was heavy rain and no visibility. The Kamikawa Maru and Sagara Maru launched 11 F1M2 and six Aichi E13A 'Jake' single-engine floatplanes. About 23 miles (37 km) west-north-west of Panjang island, an E13A1 from the Kamikawa Maru spotted a Consolidated Catalina twin-engined reconnaissance flying boat of the RAF’s No. 205 Squadron and attacked it from the rear, inflicting damage and destroying its radio. The E13A1 shadowed the Catalina for 25 minutes until five Nakajima Ki-27 'Nate' single-engined fighters from the Imperial Japanese army air force’s 1^st Sentai in Indo-China arrived and shot it down. The flying boat’s crew were the first casualties of the Pacific War. Unaware of this incident, the British took no action.

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.

At this time Thailand possessed 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 machines, 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 Indo-China. However, the Thai navy had two operational submarines, Matchanu and Wirun, which were a cause of concern for the Japanese commanders.

The Royal Thai Army started to establish new military units in the Kra peninsula. These included, in the Chumphon area, the 38^th Battalion at Ban Na Nian, Tambon Wang Mai, Muang District 5.6 miles (9 km) from the provincial capital; in the Nakhon Si Thammarat area, the 39^th Battalion at Tambon Pak Phoon, Muang District of Nakhon Si Thammarat, the 15^th Artillery Battalion at Tambon Pak Phoon, Muang District of Nakhon Si Thammarat and the headquarter of the 6^th Division at Tambon Pak Phoon, Muang District of Nakhon Si Thammarat; in the Trang area, the 40^th Battalion; at Songkla (otherwise Singora), the 5^th Battalion at Tambon Khao Kho Hong, Hat Yai District of Songkla, transferred from Bang Sue to Hat Yai by military train on 18 February 1940 as the first unit that moved to the south, the 41^st Battalion at Suan Tun, Tambon Khao Roob Chang, Muang District of Songkla and the 13^th Artillery Battalion at Suan Tun, Tambon Khao Roob Chang, Muang District of Songkla; and in the Pattani area, the 42^nd Battalion at Tambon Bo Thong, Nong Jik District of Pattani.

For its aggression to the south, the Imperial Japanese army used elements of Lieutenant General Shojiro Iida’s 15^th Army and Lieutenant General Tomoyuki Yamashita’s 25^th Army stationed in Indochina. Both armies included combat air units. The task of the 15^th Army was to invade Burma, and that of the 25^th Army was to invade Malaya and take Singapore. In order to attack Burma, the 15^th Army had to be moved over the Bangkok plain, while the 25^th Army had to attack Malaya along the isthmus of Kra. The attack through Thailand on Malaya and Singapore had been planned by Colonel Masanobu Tsuji while he was part of the No. 82 Section. In all, the Japanese had about 100,000 men who needed to pass through Thailand.

The invasions received naval support from Admiral Nobutake Kondo’s 2^nd Fleet, which also covered the landings in Thailand and at Kota Baru in Malaya. Japanese ships known to have been involved in the undertaking were the light cruiser Kashii, which escorted seven transport vessels carrying troops of the 143^rd Regiment from Saigon in Indo-China, the destroyers Asagiri, Amagiri, Sagiri, Yugiri, Shirakumo and Shinonome, which supported the 25^th Army's landings in southern Thailand; the escort ship Shimushu, which escorted the transport vessels Zenyo Maru, Miike Maru and Toho Maru to Nakorn Sri Thammarat in southern Thailand with more of the 143^rd Regiment; and the merchant seaplane carriers Kamikawa Maru and Sagara Maru.

In all, 18 transport vessels were involved, this total which including three ships scheduled to land troops at Kota Baru.

Japanese troops invaded Thailand by land from Indo-China and by sea with landings to the south of Bangkok and at various points along the isthmus of Kra 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 a time late in the morning.

The 15^th Army had a number of objectives. In Phra Tabong province, at dawn Lieutenant General Takuma Nishimura’s Imperial Guards Division and Lieutenant General Hiroshi Takeuchi 55^th Division crossed the border from Indo-China into Thailand’s recently reclaimed Phra Tabong province at Tambon Savay Donkeo, Athuek Thewadej District of Battambang. The Japanese encountered no resistance, and from Sisophon swung north-west into Aranyaprathet (then still a district of Prachinburi province) along the nearly finished railway link between Aranyaprathet and Monkhol Bourei. which was opened for traffic on 11 April 1942.

In Chumphon province, the 1/143^rd Regiment of the 55^th Division landed at Chumphon on the morning of 8 December from two troop ships. The landed troops established a beach-head but were pinned by the determined resistance of the Thai Yuwachon Thahan (cadets of the 52^nd Yuwachon Thahan Training Unit from Sriyaphai Secondary School), along with the regular army’s 38^th Battalion and Provincial Police of Chumphon. Fighting ended in the afternoon when the Thais received orders to cease fire. By this time the Thais had lost Captain Thawin Niyomsen, commander of the 52^nd Yuwachon Thahan Unit, a number of provincial policemen and a few civilians.

Nakhon Si Thammarat was the location of the Thai 6^th Division’s headquarters and 39^th Battalion. Three Japanese troopships, Zenyo Maru, Miike Maru and Toho Maru, were earmarked to land troops at Nakhorn Sri Thammarat, covered by Shimushu. The ships anchored a short distance off the coast during the night of 7/8 December. The ships carried 1,510 men and 50 trucks of the 3/143^rd Regiment, the 18^th Air District Regiment and an army air force signals unit, the 32^nd Anti-Aircraft Battalion and the 6^th Labour Construction Company. Shortly after 00.00, the ships began disembarking their troops at Tha Phae canal, which was also known as the Pak Phoon canal, to the north of Camp Vajiravudh. The landing was just beside the main Thai army camp, Camp Vajiravudh. The Thais, forewarned by the earlier Japanese invasion at Songkhla, immediately went into action. The battle lasted until 12.00, when the prime minister’s orders for a ceasefire were received.

Prachuap Khiri Khan was home to the Royal Thai air force’s 5^th Wing under the command of Wing Commander Mom Luang Prawat Chumsai. The 2/143^rd Regiment, under the command of Major Kisoyoshi Utsunomiya, landed at 03.00 from one troop ship, and occupied the town after crushing the police resistance there. Further landings were made near the airfield to the south. The Japanese besieged to the airfield, but the Thai airmen along with elements of the Prachuap Khiri Khan provincial police managed to hold out until 12.00 on the next day, when they too received the ceasefire order.

The 3/4^th Guards Regiment landed at Samut Prakan in the early hours of 8 December. The battalion was tasked with the capture of Bangkok, and was met by a small Thai police detachment. Despite a tense confrontation, there was no fighting and the Japanese subsequently agreed not to enter the Thai capital until formal negotiations were concluded.

Japanese Aircraft bombed Bangkok, one bomb falling on the main post office but failed to detonate. While police rounded up Japanese residents, the Thai cabinet debated its options as its members waited for the prime minister to arrive. Some favoured continued resistance, including the establishment of a government-in-exile, but when Phibun finally arrived it was decided to agree to Japan’s demands. Japanese troops then moved into Bangkok, occupying Sampeng ('Chinatown') and turning the Chamber of Commerce building into a command post.

Japanese aircraft attacked the Don Muang air force base, which was defended by the Thai air force. The Thais lost six fighter aircraft to a numerically superior Japanese force.

An infantry company of the 1/143^rd Regiment landed from one troop ship at the coastal village of Ban Don in the early hours of 8 December. The Japanese marched into Surat Thani, where they were opposed by Thai police and civilian volunteers. There was desultory fighting during a rainstorm, and this ended only in the afternoon when the hard-pressed Thais received orders to lay down their arms. The Thais lost 17 or 18 dead, but the number of injured is not known.

Southern Thailand and northern Malaya were the tasked of the 25^th Army. As a result of its proximity to the Malayan border, Pattani was the 25^th Army's second most important objective, and here eight destroyers, including Shirakumo and Shinonome, provided support for the five troop transports. The landings by Major Shigehara Asaeda’s 42^nd Regiment of Lieutenant General Matsui Takura’s 5^th Division were made despite the roughness of the sea and on unsuitable unsuitable beaches: when a member of the No. 82 Unit before the war, Asaeda had been involved with intelligence-gathering in Burma, Thailand and Malaya, and had selected Pattani as a suitable landing site. Unknown to him, however, in front of the sandy beach was a muddy sea bed which caused the invading force considerable difficulty. The invaders were effectively opposed by the Thai army’s 42^nd Battalion, the Pattani provincial police and Thai Yuwachon Thahan units (the 66^th Yuwachon Thahan Training Unit from Benjama Rachoothit School), until the battalion was ordered to cease fire at 12.00. The Thai battalion commander, Khun Inkhayutboriharn, was killed in action along with 23 other ranks, five provincial policemen, fout Yuwachon Thahan members, and five civilians.

The port city of Songkhla was one of the main objectives of the 25^th Army. During the early hours of 8 December, three regiments of Takuro’s 5^th Division, under the command of Colonel Tsuji, landed from 10 troop transports. The landing was supported by the destroyers Asagiri, Amagiri, Sagiri and Yugiri. The Thai garrison at Khao Khor Hong (the 41^st Battalion and 13^th Artillery Battalion) immediately occupied positions alongside the roads leading south toward Malaya, but were brushed aside into positions the main Japanese advance could ignore. A further clash occurred at Hat Yai. The Thais lost 15 men killed and between 30 and 55 men wounded before the fighting ceased at 12.00 in response to orders for an armistice to be arranged.

While these events were taking place in Thailand, troops of the 25^th Army also landed farther to the south at Kota Bharu in Malaya.

Once Thailand had been secured, the 15^th Army's 143^rd Regiment was moved to the north to replace the Imperial Guards Division, which then headed south to join the 25^th Army for the invasion of Malaya and conquest of Singapore. The 15^th Army moved to the positions from which it would undertake the Japanese 'B' (iii) invasion of Burma.

Phibun’s decision to sign an armistice with Japan effectively ended Churchill’s hopes of forging an alliance with Thailand. Phibun also granted Japan permission to use Thailand as a base of operations for the continuing 'E' (i) invasion of Malaya. Within hours of the armistice’s implementation, squadrons of Japanese aircraft arrived on Songkla airfield from Indo-China, making it possible for them to carry out longer-range attacks on strategic bases in Malaya and Singapore.

On 14 December, Phibun signed a secret agreement with the Japanese committing Thai troops to the Malayan and Burmese campaigns. An alliance between Thailand and Japan was formally signed on 21 December, and on 25 January 1942, Thailand declared war on the USA and UK. In response, all Thai assets in the USA were frozen by the federal government. While the Thai ambassador in London delivered the declaration of war to the British administration, Seni Pramoj, Thai ambassador in Washington, DC, refused to do so, instead embarking on the creation of the Free Thai Movement.