Invading Siamese forces attempt to exploit the political chaos in Vietnam, but are ambushed and annihilated at the Mekong river by the Tây Sơn in the Battle of Rạch Gầm-Xoài Mút.

The Rattanakosin Kingdom, also known as the Kingdom of Siam after 1855, refers to the Siamese kingdom between 1782 and 1932. It was founded in 1782 with the establishment of Rattanakosin (Bangkok), which replaced the city of Thonburi as the capital of Siam. This article covers the period until the Siamese revolution of 1932.

Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of mainland Southeast Asia, with an area of about 331,000 square kilometres (128,000 sq mi) and a population of over 100 million, making it the world's fifteenth-most populous country. One of two communist states in Southeast Asia, Vietnam shares land borders with China to the north, and Laos and Cambodia to the west. Its capital is Hanoi and its largest city is Ho Chi Minh City.

The Mekong or Mekong River is a transboundary river in East Asia and Southeast Asia. It is the world's twelfth-longest river and the third-longest in Asia with an estimated length of 4,909 km (3,050 mi) and a drainage area of 795,000 km² (307,000 sq mi), discharging 475 km³ (114 cu mi) of water annually. From its headwaters in the Tibetan Plateau, the river runs through Southwest China, Myanmar, Laos, Thailand, Cambodia, and southern Vietnam. The extreme seasonal variations in flow and the presence of rapids and waterfalls in the Mekong make navigation difficult, though the river remains a major trade route between Tibet and Southeast Asia. The construction of hydroelectric dams along the Mekong in the 2000s through the 2020s has caused serious problems for the river's ecosystem, including the exacerbation of drought.

The Tây Sơn dynasty (Vietnamese: [təj ʂəːn]; Vietnamese: "Nhà Tây Sơn" or "Triều Tây Sơn",, officially Đại Việt, was an imperial dynasty of Vietnam. It originated in a revolt led by three peasant brothers with the surname Nguyễn, rebelling against the Lê dynasty, Trịnh lords and Nguyễn lords. The Tây Sơn would later be succeeded by the Nguyễn dynasty.