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Between 1539 and 1855 Burma and Siam (present-day Thailand) fought some two dozen wars. Their long-running conflict was one of the bitterest rivalries in military history, yet it remains largely unknown in the West.

The Burmese instigated most of the wars with incursions into the fertile lands of the Mon people just north of Siam. These attacks often preceded full-scale invasions south into central Siam via Three Pagodas Pass and four other passes that access the valley and village of Kanchanaburi. The armies also clashed up north around Chiang Mai, but the decisive battles in the Burmese-Siamese wars usually raged around Kanchanaburi, at the confluence of the Khwae Noi and Khwae Yai rivers—brought to popular attention in the 20th century by the novel and subsequent war film The Bridge on the River Kwai.

In 1766 a 60,000-man Burmese army advanced on Three Pagodas Pass, which was defended by some 3,000 Siamese troops. Much like 300 Spartans held out against tens of thousands of Persians for three days at the 480 bc Battle of Thermopylae, so too the hard-pressed Siamese held Three Pagodas Pass for several days. Their stubborn resistance prompted the Burmese general to send most of his army through the other passes surrounding Kanchanaburi. King Ekkathat of Siam’s Ayutthaya Kingdom chose not to engage the Burmese army as it gathered on the central plain below the passes. He instead withdrew the bulk of his army east from Kanchanaburi to the capital city of Ayutthaya, hoping the monsoon rains would force the Burmese to suspend the invasion. But the besieging army outlasted the misery of the wet months and in April 1767 sacked the starving, disease-ridden capital. Only a Chinese invasion of Burma later that year forced the occupiers to withdraw.

In 1784 the Siamese successor to the throne, King Rama I, received word from spies that Burma was again preparing an invasion. Burmese King Bodawpaya planned to advance through Kanchanaburi with an unprecedented 144,000 well-armed and highly trained troops organized into nine armies, which would surge through the five passes and form up on the central plain for a decisive final battle. Rama and his brother, General Maha Surasinghanat, knew their 70,000-man army would not be able to stop the Burmese if they made it down onto the plain. Resolving to meet the Burmese in the passes, Rama organized five armies of 12,000 troops—some bearing muskets, though most armed with swords—each with a support unit of smoothbore cannon and war elephants carrying musketeers and smaller swivel-mounted cannon. The brothers would command a reserve of 10,000 elite warriors ready to plug any holes that might develop in the lines.

While we don’t know the exact day in January 1786 the invasion came, we do know that in speeches to their troops on the eve of battle both Rama and his brother stressed that Siam was facing its greatest peril. They told the men they would need to band together as what Eastern military theorists dubbed a “death army”—a force willing to fight to the bitter end without regard for personal survival. The troops spent the night at Buddhist and Hindu shrines, burning incense to their ancestors and preparing themselves for almost certain death.

Just after dawn Bodawpaya sent his armies into the passes, each division led by war elephants and troops clad in body armor. They ran headlong into Siamese formations, and soon thousands of men were engaged in hand-to-hand combat, slashing at each other with swords and firing muskets and cannon at point-blank range. War elephants went down in bellowing agony, randomly crushing men of both armies to death.

Amid the carnage the Siamese slowly pushed the invaders back down the passes, the decisive moment coming when Rama led the reserve force into Three Pagodas Pass and routed the Burmese. In the hours that followed the Siamese killed or wounded an estimated 70,000 Burmese while losing 15,000 of their own. Few of the more than 1,000 war elephants that participated in the battle survived. Corpses littered the passes, and the waterways around Kanchanaburi ran red with blood.

Rama I and his “death army” saved Siam, and Burma never again posed a major threat to the country that has been known as Thailand since 1949. To commemorate the people’s greatest victory, the Royal Thai Army started construction on the Nine-Army Battle Historical Park in 1999. The park visitor center, 25 miles north of Kanchanaburi, features dioramas of the various phases of the battle and examples of period weaponry and armor. Park attendants often suggest visitors leave the grounds before sunset, lest they encounter the ghosts of those killed in battle—advice most locals heed. 

Freelance writer and author Steven M. Johnson teaches at Paññasastra University in Phnom Penh, Cambodia.