The Burmese–Siamese War (1785–1786), known as the Nine Armies' Wars () in Siamese history because the Burmese came in nine armies, was the first war between the Konbaung dynasty of Burma and the Siamese Rattanakosin Kingdom of the Chakri dynasty.

King Bodawpaya of Burma pursued an ambitious campaign to expand his dominions into Siam. In 1785, three years after the foundation of Bangkok as the new royal seat and the Chakri dynasty, King Bodawpaya of Burma marched massive armies with total number of 144,000 to invade Siam in nine armies through five directions including Kanchanaburi, Ratchaburi, Lanna, Tak, Thalang (Phuket), and the southern Malay Peninsula. However, the overstretched armies and provision shortages deemed the Burmese campaign failed. The Siamese under King Rama I and his younger brother Prince Maha Sura Singhanat successfully warded off Burmese invasions. By early 1786, the Burmese had largely retreated.

After the truce during the rainy season, King Bodawpaya resumed his campaign in late 1786. King Bodawpaya sent his son Prince Thado Minsaw to concentrate his forces on Kanchanaburi in only a single direction to invade Siam. The Siamese met the Burmese at Tha Dindaeng, hence the term "Tha Din Daeng campaign". The Burmese were again defeated and Siam managed to defend its western border. These two failed invasions ultimately turned out to be the last full-scale invasion of Siam by Burma.

Background Traditional rivalries and dispute over Mon rebels and the Tenasserim coast led to the Burmese-Siamese wars in the eighteenth century. In 1767, the Burmese of the newly founded Konbaung dynasty invaded and destroyed the Ayutthaya Kingdom. King Taksin of Thonburi reunited Siam in the aftermath of the destruction of the Ayutthaya government and capital. In 1774, most of the Lanna Kingdom, which had been under the Burmese rule for about 250 years, came under Siamese domination with exception of Chiang Saen still under Burmese rule. During the Burmese–Siamese War (1775–1776), Siam nearly succumbed to the Burmese forces led by General Maha Thiha Thura and Siam's manpower was depleted.

In 1782, Chao Phraya Chakri was crowned as the King Rama I of Siam and founded the Chakri dynasty. In the same year, Prince Badon Min dethroned King Phaungkaza Maung Maung and crowned himself as King Bodawpaya. King Bodawpaya began his reign with glorious military conquests. In 1784, he sent his son Prince Thado Minsaw to successfully conquer the Kingdom of Mrauk U or the Arakan Kingdom. After Arakan, King Bodawpaya turned his eyes on Siam as his next military expedition.

Nine Armies War (July 1785 – March 1786) Burmese preparations King Bodawpaya began his Siamese expeditions in July 1785. The Burmese forces were drafted from its various tributary states, most notably the northern Shan states of Hsenwi, Mongnai, Kengtung, Banmaw, Mongkawng and Hsipaw. Bodawpaya first sent Mingyi Mingaung Kyaw to lead an army of 10,000 men to Martaban to arrange the provisions and supplies for the massive royal army and for the fleets at Mergui. However, supply shortage and line communications were the main issue for the Burmese armies. The Burmese were unable to quickly move the large number of men and provisions in time.

King Bodawpaya and his armies left Ava on November 11, 1785. When the king arrived at Martaban on December 20, he found that the supplies did not meet the demands. He ordered Mingyi Mingaung Kyaw arrested from Mergui to be brought to him in chains. After waiting for four days at Martaban, King Bodawpaya was furious that the transportation of men, horses and elephants across the Salween River was delayed and had not been completed. He struck a spear at one of his generals, wounding him. Only with the beseeching of the governor of Kawthanti that the royal temper was cooled down. After staying in Martaban for a month, King Bodawpaya and his royal forces left Martaban on January 19, 1786, towards the Three Pagodas Pass.

Siamese preparations In November 1785, three Burmese men were captured by the Karens of Kyaukkaung. The captured Burmese revealed to Siamese authorities that King Bodawpaya was planning a massive invasion of Siam in multiple directions. King Rama I convened a council of royal princes and ministers to discuss the situation. The Siamese then sent a Burmese man named Nga Gan, who was a former retainer of King Bodawpaya captured by the Siamese, to negotiate with King Bodawpaya at the Three Pagodas. King Bodawpaya, however, was not interested in peace-making and instead inquired Nga Gan about Siamese preparations. The Bangkok court then sent warnings to various cities in Northern and Southern Siam about the impending invasions.

Nine Armies War: Order of battle

Order of battleBurmeseSiameseKing Bodawpaya organizes his armies to invade Siam in five different directions. Thai sources, however, counted the Burmese armies as nine armies. The total massive number of Burmese armies was 144,000 men. Here is the organization of the Burmese armies following the traditional Thai narrative;While supply shortage was the problem for the Burmese, lack of manpower was the main concern for the Siamese. Twenty years of nearly continuous warfare left Siam's manpower depleted and the population reduced. The Bangkok court only managed to assemble the total force of 70,000 men about a half of total number on the Burmese side. The initial defense strategy was to concentrate the forces on critical points including the western and northern fronts, while the southern front and the Malay peninsula was left mostly undefended. The 1st Division had 10,000 men under the command of Mingyi Mingaung Kyaw. It was stationed at Myeik and were planned to take the southern provinces of Thailand. Mingyi Mingaung Kyaw was also tasked with provision supply for the royal troops, however, King Bodawpaya ordered Mingyi Mingaung Kyaw to arrested for his failure to provide adequate supplies. The division was taken over by Maha Thiri Thihathu (called "Kinwun Mingyi" in Thai sources). Later Mingyi Mingaung Kyaw was brought to King Bodawpaya, currently stationed in the Three Pagodas Pass and executed on the spot. The 2nd Division had 10,000 men under the command of Nemyo Nawrahta (called "Anaukpet Taik Wun" in Thai sources) stationed at Tavoy. It was tasked to invade Ratchaburi and Phetchaburi. Although listed as a single division, it was divided into 3 different groups Dawei Wun was ordered to lead a force of 3,000 men through the Chao Khawao pass in Suan Phueng Nemyo Nawrahta himself led a force of 4,000 men through the Bong Ti pass The remaining 3,000 men were ordered to join up with the 1st Division in Myeik. The 3rd Division had 30,000 men under Prince Thado Thiri Maha Uzana, the king's brother, stationed at Chiang Saen. It was tasked to invade Lampang, the Lanna Kingdom and Northern Siam as a whole. The Main Royal Army of King Bodawpaya would march from Mawlamyine to invade Siam at Kanchanaburi through the Three Pagodas Pass. The 4th Division had 10,000 men under the command of Minhla Kyawdin as the first vanguard. The 5th Division had 5,000 men under the command of Mingyi Maha Mingaung as the second vanguard. The 6th Division had 12,000 men under the command of Prince Thiri Damayaza (known as Takin Gama) the king's third son The 7th Division had 11,000 men under the command of Prince Thado Minsaw (known as Takin Sagu) the king's second son. The 8th Division had 50,000 men directly under royal command of King Bodawpaya. The 9th Division had 6,000 men under the command of Nawrahta Kyawgaung. It was tasked to capture Tak and invade Northern Siam.Kanchanaburi front: Prince Maha Sura Singhanat (known in Burmese sources as Einshe Paya Peikthalok) of the Front Palace, the king's younger brother, would lead armies of 30,000 men to fend off the main armies of King Bodawpaya coming into Kanchanaburi, west of Bangkok. The prince was accompanied by his two top generals Phraya Kalahom Ratchasena and Phraya Chasaenyakorn. He was also accompanied by his half-younger brother Prince Chakchetsada and Chao Phraya Rattanapipit the Samuha Nayok. Ratchaburi front: Chao Phraya Thamma Boonrot led an army to 5,000 men at Ratchaburi to defend against the Burmese troops coming from Tavoy. Northern front: Prince Anurak Devesh, the king's nephew, would lead armies of 15,000 men to station at Nakhon Sawan to receive the Burmese armies coming from the north. The prince was accompanied by Chao Phraya Mahasena Pli the Samuha Kalahom and Phraya Phraklang Hon. King Rama I himself led an army of 20,000 men stationed at Bangkok to provide relief forces for any fronts in critical conditions.

From Martaban, Bodawpaya marched his armies through the Three Pagodas Pass. He ordered fourth and fifth divisions to enter through the Three Pagodas first into Kanchanaburi, followed by sixth and seventh divisions of his two sons and then the king's own army settled in Ranti (Alante) River. The vanguard of Minhla Kyawdin and Mingyi Maha Mingaung headed for the old city of Kanchanaburi (known today as "Latya", about fifteen kilometers to the northwest of modern city of Kanchanaburi), while the two princes stationed at Tha Dindaeng and Samsop in modern Sangkhlaburi District.

Kanchanaburi front

Prelude

In December 1785, Prince Maha Sura Singhanat, the Uparat (Viceroy) of Siam, led an army of 30,000 men out of the capital to confront the Burmese invasion. By royal decree, Phraya Kalahom Ratchasena and Phraya Chasaenyakorn were appointed to command the vanguard, while Chao Phraya Rattanapipit (Son) served as the kiakkai (camp marshal) and Prince Chakchetsada acted as the yokkabat (military auditor). The Siamese forces advanced to Tambon Lat Ya in Kanchanaburi, situated at the foot of the Banthat Range. There, the Prince ordered the army to establish a defensive position on the Lat Ya plain, featuring a camp with interconnected "wings" designed to maintain a continuous line of defense. Additionally, Binnya Sein (Phraya Mahayotha) was commanded to lead 3,000 Mon troops to intercept the Burmese at Dan Kram Chang, located in what is now Erawan National Park.

The main invasion force under King Bodawpaya entered Kanchanaburi through the Three Pagodas Pass. The 4th Burmese column, led by Myin Wun, advanced via Sai Yok and crossed the Khwae Yai River at Tha Kradan.Damrong Rajanubhab, Prince. Phongsawadan Rueang Rao Rop Phama Khrang Krung Thon Lae Krung Thep ("History Regarding Our Wars with the Burmese during Krung Thon and Krung Thep") Following the river downstream to Dan Kram Chang, Myin Wun's 10,000-man force routed the Mon contingent under Phraya Mahayotha and subsequently established a camp at Lat Ya. This force was later bolstered by the 5,000-man 5th column under Myinmawun, bringing the total Burmese strength at Lat Ya to 15,000 troops.

Other segments of the Burmese army were positioned strategically: the 6th column under Prince Thiri Damayaza at Tha Din Daeng; the 7th column under Prince Thado Minsaw at Sam Sop; and the 8th column—the main royal army of King Bodawpaya—at the Ranti River. The Siamese attempted to negotiate with Bodawpaya at the Ranti River by sending Nga Kan (or Na Khan), a captive from the War of Athi Wungyi, as an envoy, but the mission was unsuccessful. Despite their strategic positions, the Burmese forces suffered from severe logistical failures and provision shortages. The large host was unable to secure enough local supplies, and the difficulty of transporting provisions from distant regions significantly hampered the vanguard's operations.

Battle of Thung Lat Ya