The Burmese–Siamese War was the first military conflict between the Konbaung Dynasty of Burma (Myanmar) and the Ban Phlu Luang Dynasty of Siam (Thailand). It reignited the centuries-long warfare between the two states that would go on for another century. The Burmese were "on the brink of victory" when they suddenly withdrew from their siege of Ayutthaya because their king Alaungpaya had fallen ill. He died three weeks later, ending the war.
The casus belli were over the control of the Tenasserim coast and its trade, and the Siamese support for ethnic Mon rebels of the fallen Restored Hanthawaddy Kingdom. The newly founded Konbaung Dynasty had wanted to reestablish Burmese authority in the upper Tenasserim coast (present-day Mon State) where the Siamese had provided support to the Mon rebels and deployed their troops. The Siamese had refused Burmese demands to hand over the rebel Mon leaders or to stop their intrusions into what the Burmese considered their territory.
The war began in December 1759 when 40,000 Burmese troops led by Alaungpaya and his son Hsinbyushin invaded down the Tenasserim coast from Martaban (Mottama). Their battle plan was to go around the heavily defended Siamese positions along shorter, more direct invasion routes. The invasion force overran relatively thin Siamese defenses in the coast, crossed the Tenasserim Hills to the shore of the Gulf of Siam, and turned north towards Ayutthaya. Taken by surprise, the Siamese scrambled to meet the Burmese in their south, and put up spirited defensive stands en route to Ayutthaya. But battle-hardened Burmese forces overcame numerically superior Siamese defenses and reached the outskirts of Siamese capital on 11 April 1760. But only five days into the siege, the Burmese king suddenly fell ill and the Burmese command decided to withdraw. An effective rearguard operation by Gen. Minkhaung Nawrahta allowed for an orderly withdrawal.
The war was inconclusive. While the Burmese regained control of the upper coast down to the Tavoy (Dawei), they had not eliminated the threat to their hold on the peripheral regions, which remained tenuous. They were forced to deal with Siamese-supported ethnic rebellions in the coast (1762, 1764) as well as in Lan Na (1761–1763). The Burmese would launch their next invasion in 1765, and topple the four-century-old Ayutthaya Kingdom in 1767.