Post-Angkor period
The post-Angkor period of Cambodia, also called the Middle period, refers to the historical era from the early 15th century to 1863, the beginning of the French protectorate of Cambodia. As reliable sources are very rare, a defensible and conclusive explanation that relates to concrete events that manifest the decline of the Khmer Empire, recognised unanimously by the scientific community, has so far not been produced. However, most modern historians have approached a consensus in which several distinct and gradual changes of religious, dynastic, administrative and military nature, environmental problems and ecological imbalance coincided with shifts of power in Indochina and must all be taken into account to make an interpretation. In recent years scholars' focus has shifted increasingly towards human–environment interactions and the ecological consequences, including natural disasters, such as flooding and droughts.
Stone epigraphy in temples, which had been the primary source for Khmer history, is already a rarity throughout the 13th century, ends in the third decade of the fourteenth, and does not resume until the mid-16th century. Recording of the Royal Chronology discontinues with King Jayavarman IX Parameshwara, who reigned from 1327 to 1336. There exists not a single contemporary record of even a king’s name for over 200 years. Construction and maintenance of monumental temple architecture had come to a standstill after Jayavarman VII's reign. According to author Michael Vickery there only exist external sources for Cambodia’s 15th century, the Chinese Ming Shilu and the earliest Royal Chronicle of Ayutthaya, which must be interpreted with greatest caution.
The single incident which undoubtedly reflects reality, the central reference point for the entire 15th century, is a Siamese intervention of some undisclosed nature at the capital Yasodharapura around the year 1431. Historians relate the event to the shift of Cambodia's political centre southward to the river port region of Phnom Penh and later Longvek.
Sources for the 16th century are more numerous, although still coming from outside of Cambodia. The kingdom's new capital was Longvek, on the Mekong, which prospered as an integral part of the 16th century Asian maritime trade network, via which the first contact with European explorers and adventurers occurred. The rivalry with the Ayutthaya Kingdom in the west resulted in several conflicts, including the Siamese conquest of Longvek in 1594. The Vietnamese southward expansion reached Prei Nokor/Saigon at the Mekong Delta in the 17th century. This event initiates the slow process of Cambodia losing access to the seas and independent marine trade.
Siamese and Vietnamese dominance intensified during the 17th and 18th century, provoking frequent displacements of the seat of power as the Khmer monarch's authority decreased to the state of a vassal. Both powers alternately demanded subservience and tribute from the Cambodian court. In the mid 19th century, with dynasties in Siam and Vietnam firmly established, Cambodia was placed under joint suzerainty between the two regional empires, thereby the Cambodian kingdom lost its national sovereignty. British agent John Crawfurd states: "...the King of that ancient Kingdom is ready to throw himself under the protection of any European nation..." To save Cambodia from being incorporated into Vietnam and Siam, King Ang Duong agreed to colonial France's offers of protection, which took effect with King Norodom Prohmbarirak signing and officially recognising the French protectorate on 11 August 1863.
Historical background and causes
The Khmer Empire had steadily gained hegemonic power over most of mainland Southeast Asia since its early days in the 8th and 9th centuries. Rivalries and wars with its western neighbour, the Pagan Kingdom of the Mon people of modern-day Burma were less numerous and decisive than those with Champa to the east. The Khmer and Cham Hindu kingdoms remained for centuries preoccupied with each other's containment and it has been argued that one of the Khmer's military objectives was "...in the reigns of the Angkor kings Suryavarman II and Jayavarman VII." the conquest of the Cham ports, "...important in the international trade of the time". Even though the Khmer suffered a number of serious defeats, such as the Cham invasion of Angkor in 1177, the empire quickly recovered, capable to strike back, as it was the case in 1181 with the invasion of the Cham city-state of Vijaya.Mongol incursions into southern China and political and cultural pressure caused the southward migration of the Tai people and Thai people and their settling on the upper Chao Phraya River in the 12th century. The Sukhothai Kingdom and later the Ayutthaya Kingdom were established and "...conquered the Khmers of the upper and central Menam valley and greatly extended their territory..."
Military setbacks
Although a number of sources, such as the Cambodian Royal Chronicles and the Royal chronicles of Ayutthaya contain recordings of military expeditions and raids with associated dates and the names of sovereigns and warlords, several influential scholars, such as David Chandler and Michael Vickery doubt the accuracy and reliability of these texts. Other authors, however, criticise this rigid "overall assessment".David Chandler states in A Global Encyclopedia of Historical Writing, Volume 2: "Michael Vickery has argued that Cambodian chronicles, including this one, that treat events earlier than 1550 cannot be verified, and were often copied from Thai chronicles about Thailand..."
Linguist Jean-Michel Filippi concludes: "The chronology of Cambodian history itself is more a chrono-ideology with a pivotal role offered to Angkor." Similarities apply to Thai chronological records, with the notable example of the Ramkhamhaeng controversy.
According to the Siamese Royal chronicles of Paramanuchitchinorot, clashes occurred in 1350, around 1380, 1418 and 1431.
Land or people debate
Siamese sources record the habit of capturing sizeable numbers of inhabitants from the capital cities and centres of civilisation of the defeated parties in Chiang Mai and Angkor which can be assumed to have accelerated the cultural decline.Author Michael Vickery debates the degree of importance of this subject in his publication "Two Historical Records of the Kingdom of Vientiane - Land or People?": "It is not at all certain that Angkor desired manpower in central Thailand, rather than simply control over the rich agricultural resources." and "...whether the political economy of early Southeast Asia resulted in rulers being more concerned with control of land or control of people..." and "...both sides of this discussion have offered ad hoc, case-by-case pronunciamentos, which are then repeated like mantra... Critical discussion of the question is long overdue..."
;Contrary views:
Author Akin Rabibhadana, who quotes Ram Khamhaeng: "One particular characteristic of the historical Southeast Asian mainland states was the lack of manpower. The need for manpower is well illustrated by events following each war between Thailand and her neighbours. The victorious side always carried off a large number of people from the conquered territory. Whole villages were often moved into the territory of the conqueror, where they were assimilated and became the population of the conqueror."
David K. Wyatt: "As much as anything else, the Tai müang was an instrument for the efficient use of manpower in a region where land was plentiful in relation to labor and agricultural technology."
Baker and Phongpaichit argues that, "War in the region was... an enterprise to acquire wealth, people, and scarce urban resources."
Bronson states, "No farmers in any region outside southern and eastern Asia could produce as much food with as little labor from the same amount of land."
And Aung-Thwin wrote: "Much of the warfare of early Southeast Asia witnessed the victor carrying off half the population of the vanquished foe and later resettling them on his own soil. Pagan was located in the dry belt of Burma, and depended mainly upon irrigated agriculture for its economic base. Land was plentiful but labor was extremely difficult to obtain."
Dynastic and religious factors
The complete transition from the early Khmer kingdom to the firm establishment of the Mahidharapura dynasty, which originated west of the Dângrêk Mountains at Phimai in the Mun river valley lasted several decades. Some historians argue, that these kings failed to acquire absolute central administrative control and had limited access to local resources. The dynasty discontinued "ritual policy" and genealogical traditions. Further momentum ensued as Mahayana Buddhism was eventually tolerated and several Buddhist kings emerged, including Suryavarman I, Rajendravarman II and Jayavarman VII.These rulers were not considered, and did not consider themselves, as divine, which lead to a shift in perception of royal authority, central power and a loss of dynastic prestige with respect to foreign rulers. Effectively the royal subjects were given permission to re-direct attention and support from the Hindu state of military dominance with its consecrated leader, the "Varman"—protector king, towards the inner-worldly alternative with the contradictory teachings of the Buddhist temple. Indravarman III adopted Theravada Buddhism as the state religion, which implied an even more passive, introverted focus towards individual and personal responsibility to accumulate merit to achieve nirvana.
Miriam T. Stark argues that competition and rivalries in royal succession, usurpers and "second grade" rulers characterised the kingdom since the 9th century. Periods of "...consolidation alternated with political fragmentation only few rulers were able to wrest control from the provincial level".
Debate remains on the progress of the imperial society as the kingdom grew and occupied foreign lands. Authors present numerous theories about the relationship between Southeast Asian kings and the populace's loyalties, nature and degree of identity, the Mandala concept and the effects of changing state-religion. Scholar Ben Kiernan highlights a tendency to identify with a universal religion rather than to adhere to the concept of a people or nation, as he refers to author Victor Lieberman in: Blood and Soil: Modern Genocide 1500-2000 "...no formal demand, that rulers be of the same ethnicity as their subjects"