There they clashed with Vietnamese forces of Ong Tin Wuang (องติเวือง, Nguyễn Lữ[note 1]) in Sadec (Sa Đéc) and captured warships, prisoners and various types of weapons, yet later returned them to the Tay Son.

According to Vietnamese records, in April 1784, an army of 30,000 troops under the Siamese generals Lục Côn and Sa Uyển was dispatched to Cambodia and prepare to attack Gia Dinh.

According to the Royal Thai Chronicles, in March 1784, a fleet with five thousand men under Chao Fa Krom Luang Thepharirak was dispatched to attack and recapture Saigon for Nguyễn Ánh.

[3][4] The Siamese-Cambodian infantry contingents under Phraya Wichitnarong attacked Sa Đéc (Piamchopsadaek), where they defeated several Tay Son detachments.

[3][4] Meanwhile, the Siamese-Nguyễn fleet under Krom Luang Thepharirak and Nguyễn Ánh finally landed in Banteay Meas (Mang Khảm, a place belonging to Hà Tiên during that time).

The Siamese-Nguyễn fleet sailed to the Bassac River (sông Hậu in Vietnamese) and stopped in Trà Tân (Wamanao, a place near Mỹ Tho).

[9] In a letter to French preacher J. Liot, Nguyễn Ánh complained about the Siamese atrocities, who robbed, raped and slaughtered unscrupulously.

[15] The Tây Sơn reinforcements led by Nguyễn Huệ[note 1] marched south from Quy Nhon and arrived in Cochinchina territory around January 1785.

Thepharirak was confident that Nguyễn Huệ was waiting for the results of negotiations, because he saw Tây Sơn warships withdrawing to Mỹ Tho orderly.

The date was fixed on January 19, 1785 (December 9 of the year Giáp Thìn in Vietnamese lunar calendar), and notified Nguyễn Ánh.

He sent Mạc Tử Sinh to Trấn Giang (Cần Thơ) to prepare a boat, which would facilitate flight in case of defeat.

[9] 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.

[16] 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.

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.