As reliable sources (for the 15th and 16th centuries, in particular) 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.

[2][3] 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[4] coincided with shifts of power in Indochina and must all be taken into account to make an interpretation.

According to author Michael Vickery there only exist external sources for Cambodia’s 15th century, the Chinese Ming Shilu ("Veritable Records") and the earliest Royal Chronicle of Ayutthaya,[12] which must be interpreted with greatest caution.

[13] 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 (Angkor Thom) around the year 1431.

The kingdom's new capital was Longvek, on the Mekong, which prospered as an integral part of the 16th century Asian maritime trade network,[16][17] via which the first contact with European explorers and adventurers occurred.

[20] 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.

[25] The Sukhothai Kingdom and later the Ayutthaya Kingdom were established and "...conquered the Khmers of the upper and central Menam [Chao Phraya River] valley and greatly extended their territory..."[26] Although a number of sources, such as the Cambodian Royal Chronicles and the Royal chronicles of Ayutthaya[27] 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.

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.

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..."[citation needed] Author Akin Rabibhadana, who quotes Ram Khamhaeng: "One particular characteristic of the historical Southeast Asian mainland states was the lack of manpower.

"[41] The complete transition from the early Khmer kingdom to the firm establishment of the Mahidharapura dynasty (first king Jayavarman VI, 1080 to 1107), which originated west of the Dângrêk Mountains at Phimai in the Mun river valley[42] lasted several decades.

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 (c. 1295-1308) adopted Theravada Buddhism as the state religion,[44] which implied an even more passive, introverted focus towards individual and personal responsibility to accumulate merit to achieve nirvana.

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 "[local courts make]...no formal demand, that rulers be of the same ethnicity as their subjects"[47][48] Historians increasingly maintain the idea that decline was caused by progressing ecological imbalance of the delicate irrigation network and canal system of "...a profoundly ritualized, elaborate system of hydraulic engineering..."[49] at Angkor's Yasodharapura.

[50] Permanent monument construction projects and maintenance of temples instead of canals and dykes put an enormous strain on the royal resources and drained thousands of slaves and common people from the public workforce and caused tax deficits.

[51] Author Heng L. Thung addressed common sense in "Geohydrology and the decline of Angkor" as he sums things up: "...the preoccupation of the Khmers with the need to store water for the long dry season.

[8] Some historians have argued that an important reason for the Angkor court's move to the lower Mekong Delta was due to the growth of international maritime trade with the rest of the world.

Thus, it controlled the river commerce of the Khmer heartland, upper Siam and the Laotian kingdoms with access, by way of the Mekong Delta, to the international trade routes that linked the Chinese coast, the South China Sea, and the Indian Ocean.

David P. Chandler: "The 1747 inscription is the last extensive one at Angkor Wat and reveals the importance of the temple in Cambodian religious life barely a century before it was "discovered" by the French.

By the late sixteenth and early seventeenth centuries, Longvek maintained flourishing communities of Chinese, Indonesians, Malays, Japanese, Arabs, Spaniards, English, Dutch and Portuguese traders.

[61][62] Christian missionary activities began in 1555 with Portuguese clergyman friar Gaspar da Cruz,[63] the first to set foot in the Kingdom of Cambodia, who "...wasn’t able to spread the word of God and he was seriously ill[sic]."

Their sons, born and raised in Lan Xang, were alienated and while "moderately" manipulated, engaged in local court politics with the exiled Cambodians in Ayutthaya and had the ruling vassal King Ram I, who was of lower birth, killed with the help of Spanish and Portuguese sailors.

[85] Henri Mouhot: "Travels in the Central Parts of Indo-China" 1864 "Udong, the present capital of Cambodia, is situated north-east of Komput, and is four miles and a half from that arm of the Mekon which forms the great lake...Every moment I met mandarins, either borne in litters or on foot, followed by a crowd of slaves carrying various articles; some, yellow or scarlet parasols, more or less large according to the rank of the person; others, boxes with betel.

On this side were numerous processions to the pagoda, marching to the sound of music; there, again, was a band of ecclesiastics in single file, seeking alms, draped in their yellow cloaks, and with the holy vessels on their backs....The entire population numbers about 12,000 souls.

[93] The story of a Cambodian king falling in love with a Vietnamese princess, who requested and obtained Kampuchea Krom, the Mekong Delta for Vietnam is folklore, dismissed by scholars and not even mentioned in the Royal Chronicles.

[94][95] In the process of re-interpretation of the royal records and their rather doubtful contents, Michael Vickery again postulates that future publications take these contradicting facts into account: "First, the very concept of a steady Vietnamese "Push to the South" (nam tiến) requires rethinking.

[111] Admiral Léonard Charner proclaimed the formal annexation of three provinces of Cochinchina into the French Empire on 31 July 1861,[112] the beginning of the colonial era of France in South-East Asia.

France's interference in Indochina was thus a fact and the colonial community pressing to establish a commercial network in the region based on the Mekong river, ideally linking up with the gigantic market of southern China.

"[115] To save the kingdom's national identity and integrity, King Ang Duong initiated secret negotiations in a letter to Napoleon III seeking to obtain some agreement of protection with France.

Reflection of results obviously requires time, as in an article of the US National Academy of Sciences of the year 2010, the author complains: "Historians and archaeologists have, with a few notable exceptions only rarely considered the role played by environment and climate in the history of Angkor".