The Tây Sơn reinforcements led by Nguyễn Huệ marched south from Quy Nhon and arrived in Cochinchina territory around January 1785.
Huệ set up his headquarter in Mỹ Tho, not far from Trà Tân, the headquarter of the Siamese troops.

Small groups of Tây Sơn navy harassed Siamese fortified points during high tide and withdrew during low tide.
They gathered intelligence about the Siamese navy and pretended to be vulnerable.
After many victories, the Siamese army and naval forces were overconfident.
Nguyễn Huệ noticed it, and decided to avoid a direct attack on a strong Siamese force.
He sent a small naval force, under a banner of truce, to offer to parley with the Siamese.
Huệ gave many treasures to Krom Luang Thepharirak (Chiêu Tăng), and requested him not to support Nguyễn Ánh.
Huệ also promised that the Tây Sơn would pay tribute to Siam.
Thepharirak received these presents.

During the negotiations, Siamese soldiers were invited to visit the warships of the Tây Sơn navy.
Nguyễn Huệ showed sophisticated weapons to them, and gave them many treasures before they returned.
Hearing that, Nguyễn Ánh suspected the Siamese to have sinister intentions.
Thepharirak had to explain to Nguyễn Ánh that it was just a stratagem.

Thepharirak was confident that Nguyễn Huệ was waiting for the results of negotiations, because he saw Tây Sơn warships withdrawing to Mỹ Tho orderly.
Thepharirak planned a surprise attack on the Tây Sơn navy.
The date was fixed on January 19, 1785 (December 9 of the year Giáp Thìn in Vietnamese lunar calendar), and notified Nguyễn Ánh.
Ánh had a presentiment that the Siamese navy would be defeated.
He sent Mạc Tử Sinh to Trấn Giang (Cần Thơ) to prepare a boat, if defeated, they were able to flee.

However, Thepharirak was overconfident, actually it was a trap set up by Huệ.
Nguyễn Huệ, anticipating a move from the Siamese, had secretly positioned his infantry and artillery along the Mekong river (Rạch Gầm-Xoài Mút area of present-day Tiền Giang province), and on some islands in the middle, facing other troops on the northern banks with naval reinforcements on both sides of the infantry positions.

On the morning of January 20, 1785, Chiêu Tăng (Thepharirak) and the Siamese main forces left Trà Tân to attack Mỹ Tho, where Hue's headquarter was located.
Only a small group of infantry led by Sa Uyển was left in Sa Đéc.
The navy of the Nguyễn lord led by Lê Văn Quân was ordered to take the lead.
When the front navy reached Rạch Gầm River, and the rear navy reached Xoài Mút River, Nguyễn Huệ's ships dashed into the unprepared Siamese troops, preventing their advance or retreat.
In the meanwhile, the Tây Sơn artillery opened fire.
 One of the secret weapons of the Tây Sơn force was the Hỏa Hổ Thần Công (Flaming Tiger Cannon), which could release a stream of fire at a very long range.

The battle ended with a near annihilation of the Siamese force, as according to Vietnamese sources all the ships of the Siamese navy were destroyed.
Chiêu Tăng (Thepharirak) and Chiêu Sương landed in the north bank of Mỹ Tho River, then at Quang Hóa, through Cambodia, and arrived in Bangkok in March 1785.
Only 2,000 to 3,000 men of the original expedition escaped.
Other survivors stole the boats of civilians and fled to Cambodia.
On February 4, 1785, Rama I received the information that the Siamese navy was defeated.
He sent a dozen ships to rescue Siamese soldiers.
There were only ten thousand survivors.

When his navy was nearly annihilated Nguyễn Ánh and a dozen men escaped to Trấn Giang (Cần Thơ) where they met Mạc Tử Sinh and went to Hà Tiên on three ships.
In Hà Tiên, Nguyễn Ánh gathered the remnants of his navy and fled to Poulo Panjang, then to Ko Kut and finally arrived in Bangkok, where he sought refuge until August 1787.
The Tây Sơn reinforcements led by Nguyễn Huệ marched south from Quy Nhon and arrived in Cochinchina territory around January 1785. Huệ set up his headquarter in Mỹ Tho, not far from Trà Tân, the headquarter of the Siamese troops.
Small groups of Tây Sơn navy harassed Siamese fortified points during high tide and withdrew during low tide. They gathered intelligence about the Siamese navy and pretended to be vulnerable. After many victories, the Siamese army and naval forces were overconfident. Nguyễn Huệ noticed it, and decided to avoid a direct attack on a strong Siamese force. He sent a small naval force, under a banner of truce, to offer to parley with the Siamese. Huệ gave many treasures to Krom Luang Thepharirak (Chiêu Tăng), and requested him not to support Nguyễn Ánh. Huệ also promised that the Tây Sơn would pay tribute to Siam. Thepharirak received these presents.
During the negotiations, Siamese soldiers were invited to visit the warships of the Tây Sơn navy. Nguyễn Huệ showed sophisticated weapons to them, and gave them many treasures before they returned. Hearing that, Nguyễn Ánh suspected the Siamese to have sinister intentions. Thepharirak had to explain to Nguyễn Ánh that it was just a stratagem.
Thepharirak was confident that Nguyễn Huệ was waiting for the results of negotiations, because he saw Tây Sơn warships withdrawing to Mỹ Tho orderly. Thepharirak planned a surprise attack on the Tây Sơn navy. The date was fixed on January 19, 1785 (December 9 of the year Giáp Thìn in Vietnamese lunar calendar), and notified Nguyễn Ánh. Ánh had a presentiment that the Siamese navy would be defeated. He sent Mạc Tử Sinh to Trấn Giang (Cần Thơ) to prepare a boat, if defeated, they were able to flee.
However, Thepharirak was overconfident, actually it was a trap set up by Huệ. Nguyễn Huệ, anticipating a move from the Siamese, had secretly positioned his infantry and artillery along the Mekong river (Rạch Gầm-Xoài Mút area of present-day Tiền Giang province), and on some islands in the middle, facing other troops on the northern banks with naval reinforcements on both sides of the infantry positions.
On the morning of January 20, 1785, Chiêu Tăng (Thepharirak) and the Siamese main forces left Trà Tân to attack Mỹ Tho, where Hue's headquarter was located. Only a small group of infantry led by Sa Uyển was left in Sa Đéc. The navy of the Nguyễn lord led by Lê Văn Quân was ordered to take the lead. When the front navy reached Rạch Gầm River, and the rear navy reached Xoài Mút River, Nguyễn Huệ's ships dashed into the unprepared Siamese troops, preventing their advance or retreat. In the meanwhile, the Tây Sơn artillery opened fire.  One of the secret weapons of the Tây Sơn force was the Hỏa Hổ Thần Công (Flaming Tiger Cannon), which could release a stream of fire at a very long range.
The battle ended with a near annihilation of the Siamese force, as according to Vietnamese sources all the ships of the Siamese navy were destroyed. Chiêu Tăng (Thepharirak) and Chiêu Sương landed in the north bank of Mỹ Tho River, then at Quang Hóa, through Cambodia, and arrived in Bangkok in March 1785. Only 2,000 to 3,000 men of the original expedition escaped. Other survivors stole the boats of civilians and fled to Cambodia. On February 4, 1785, Rama I received the information that the Siamese navy was defeated. He sent a dozen ships to rescue Siamese soldiers. There were only ten thousand survivors.
When his navy was nearly annihilated Nguyễn Ánh and a dozen men escaped to Trấn Giang (Cần Thơ) where they met Mạc Tử Sinh and went to Hà Tiên on three ships. In Hà Tiên, Nguyễn Ánh gathered the remnants of his navy and fled to Poulo Panjang, then to Ko Kut and finally arrived in Bangkok, where he sought refuge until August 1787.
Battle of Rạch Gầm-Xoài Mút