Tay-Son rebel forces had recaptured Saigon in 1783 and forced Nguyễn Ánh to flee across the river to Siam.

While in exile, Nguyễn Ánh wishes to retake Gia Định and push the Tây-Sơn rebels out.

One of Anh's generals, Chau Van Tiep, had persuaded the King Buddha Yodfa Chulaloke of Siam to provide Nguyen Anh with support troops and a small invasion force.

In mid-1784 Nguyễn Ánh, with twenty thousand to fifty thousand Siamese troops and three hundred warships, had moved through Cambodia, then east of Tonle Sap (Toh Lay Sap in Thai) and penetrated the recently annexed provinces of Annam.

Twenty thousand Siamese troops had reached Kiên Giang and another thirty thousand had landed in Chap Lap, as the Siamese advanced towards Cần Thơ.

Later that year the Siamese captured the former Cambodian province of Gia Định, where they committed atrocities against Việt settlers; this made some locals turn their support to Tây Sơn.