Myriad state polities emerged and collapsed across mainland Southeast Asia between the 9th and 15th centuries CE. Angkor, and its associated Khmer Empire, offers the most extraordinary example for its geographic scale and lifespan. Yet through the Angkorian period and afterward, neighboring polities arose and competed with each other for control over resources and populations. Most previous studies of Classical states in Mainland Southeast Asia use a single case study, and lack the regional perspective necessary to evaluate the relative impact of political and climatic factors through time. Fewer still have blended indigenous data sources like archaeology with epigraphic and external documentary data. This paper uses the Angkorian state as a focus to trace the regional development of mainland peer polities that emerged and collapsed over a six century period.