The Battle of Rạch Gầm-Xoài Mút was fought between Tây-Sơn (Vietnamese) and Siamese forces in present-day Tiền Giang Province on January 20, 1785. It is considered one of the greatest victories in Vietnamese history.

In 1783 Tây-Sơn rebel forces recaptured Saigon and forced Nguyễn Ánh to flee across the river to Siam. While in exile Nguyễn Ánh wished to retake Gia Định and push the Tây-Sơn rebels out. One of Anh's generals, Chau Van Tiep, convinced the peaceful King Buddha Yodfa Chulaloke of Siam to provide Nguyen Anh with support troops and a small invasion force.

In mid-1784 Nguyễn Ánh, with 20,000–50,000 Siamese troops and 300 warships, moved through Cambodia, then east of Tonle Sap (Toh Lay Sap in Thai) and penetrated the recently annexed provinces of Annam. 20,000 Siamese troops reached Kiên Giang and another 30,000 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.

The Tay Son reinforcements led by Nguyễn Huệ marched south from Quy Nhon and arrived in Cochin China territory soon after. Hue avoided a direct attack on a strong Siamese force at Sa Dec and tried to set up a trap. 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.

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