In the late sixteenth century, Portuguese missionaries encountered a vast city at Angkor. Abandoned for 150 years, it had been partially restored as a Buddhist piligrimage centre but many of the monuments had been given over to the forest. They wondered about its origins. Some suggested that it was the work of Trajan, others of Alexander the Great. Following the establishment of the French colonial empire in Southeast Asia three centuries later, and in the absence of any information on prehistoric societies, the civilization of Angkor was seen as the result of Indianization, whereby Indian religions, architecture, writing and language were adopted by the indigenous inhabitants. This article presents the results of twenty years of research in the upper Mun Valley of northeast Thailand, an area that was part of the Kingdom of Angkor and seat of the dynasty of two of its greatest kings, Suryavarman II and Jayavarman VII. For the first time, the entire prehistoric cultural sequence from late hunter-gatherers to the end of the Iron Age has been documented and dated. We find that after the ingress of rice farmers from southern China in the mid second millennium BC, there were two surges in social complexity. Both were coincidental with the availability of new exotic goods through exchange. The first took place in the initial Bronze Age, in the eleventh century BC, and was followed by several centuries of relative poverty in mortuary practices. The second took place in the late Iron Age and led directly to the foundation of powerful chiefdoms from which can be traced the genesis of early civilizations in Southeast Asia, including that of Angkor.