The political, dynastic, and military decline of the Khmer Empire after the 15th century, known as the Post-Angkor Period, left a power vacuum in the Mekong floodplains of central Indochina.
United under strong dynastic rule, both Siam to the west and Vietnam to the east sought to achieve hegemony in the lowland region and the Lao mountains.
[a] Eventually, territory was annexed by both powers, who conceived, maintained and supported their favorable Cambodian puppet kings.
The 19th-century establishment of French Indochina put an end to Vietnamese sovereignty and to Siamese policies of regional expansion.
Subsequent clashes of the two countries were not caused by regional rivalry, but must be viewed in the context of the 20th-century imperial policies of foreign great powers and the Cold War.