The Burmese–Siamese War (1765–1767) was the second military conflict between the Konbaung Dynasty of Burma (Myanmar) and the Ban Phlu Luang Dynasty of Siam (Thailand), and the war that ended the four-century-old Siamese kingdom. Nonetheless, the Burmese were soon forced to give up their hard-won gains when the Chinese invasions of their homeland forced a complete withdrawal by the end of 1767. A new Siamese dynasty, to which the current Thai monarchy traces its origins, emerged to reunify Siam by 1770.
This war was the continuation of the war of 1759–1760. The casus belli of this war were also the control of the Tenasserim coast and its trade, and the Siamese support for the rebels in the Burmese border regions. The war began in August 1765 when a 20,000-strong northern Burmese army invaded northern Siam, and was joined in by three southern armies of over 20,000 in October, in a pincer movement on Ayutthaya. By late January 1766, the Burmese armies had overcome numerically superior but poorly coordinated Siamese defences, and converged before the Siamese capital.
The siege of Ayutthaya began during the first Chinese invasion of Burma. The Siamese believed that if they could hold out until the rainy season, the seasonal flooding of the Ayutthayan central plain would force a retreat. But King Hsinbyushin of Burma believed that the Chinese war was a minor border dispute, and decided to continue the siege. During the rainy season of 1766 (June–October), the battle moved to the waters of the flooded plain but failed to change the status quo. When the dry season came, the Chinese launched a much larger invasion but Hsinbyushin still refused to recall the troops. In March 1767, King Ekkathat of Siam offered to become a tributary but the Burmese demanded an unconditional surrender. On 7 April 1767, the Burmese sacked the starving city for the second time in history, committing atrocities that have left a major black mark on Burmese-Thai relations to the present day. Thousands of Siamese captives were relocated to Burma.
The Burmese occupation was short-lived. In November 1767, the Chinese again invaded with their largest force yet, finally convincing Hsinbyushin to withdraw his forces from Siam. In the ensuing Siamese civil war, Taksin's Thonburi forces emerged the sole winner by mid-1770. The Burmese had also defeated a fourth Chinese invasion by December 1769.
By then, a new stalemate had taken hold. Burma had annexed the lower Tenasserim coast but again failed to eliminate Siam as the sponsor of rebellions in her eastern and southern borderlands. In the following years, Hsinbyushin was preoccupied by the Chinese threat, and did not renew the Siamese war until 1775—only after Lan Na had revolted again with Siamese support. The post-Ayutthaya Siamese leadership proved more than capable; they defeated the next two invasions (1775–1776 and 1785–1786), and annexed Lan Na in the process.