Endemic instability characterized this period in Cambodia. By mid-15th century, the Thai-oriented administration of Angkor region was overthrown by forces loyal to Phnom Penh, but by end of the 15th century, conflict developed among the new rulers.
A Khmer revival in early 17th century was followed by a Thai overlordship of Cambodia in the 18th century. From the 17th century, the traditional regions of dissidence could rely on Vietnamese support, setting in motion a whipsaw between Vietnam and Siam, with Cambodia in between. As the importance of foreign powers in Cambodian internal affairs increased, the pattern of increasing intrusion of the Vietnamese into Cambodian life was symbolized by Vietnamese activities that resulted in the sealing off of Cambodia from maritime access to the outside world. This occurred at the time that other Southeast Asian countries, especially Siam, were becoming more involved in the outside world.
Significant social changes occurred during this period: the decline in the importance of a brahmanical priestly class (which had effectively linked landholdings, control of slaves, religious practices, education, and the throne) and the increasingly widespread influence of the Thai on Cambodian life (including the transformation of the Khmer language, i.e., the replacement of Angkorean syntax with Thai syntax). The shift of the capital from the rice-growing hinterlands of northwestern Cambodia to the trade-oriented riverbanks in the vicinity of Phnom Penh took place during this period. Another important element at this time was the inability of the king to deliver protection and stability, which undermined his position in the eyes of his subjects.
Territorial expansionist activities by the Nguyen overlords of Vietnam cut off Cambodia from maritime access to the outside world and resulted in the loss of thousands of ethnic Khmer to Vietnamese rather than Cambodian control.
Relative calm with respect to invasions from Siam and Vietnam, but internal chaos from a series of coups and countercoups by rivals in the royal family.
Continued Thai pressure on Cambodia while Vietnamese powers were distracted by internal threats (the Nguyen dealing with a populist rebellion led by the Tay Son brothers).
