The oldest empire in mainland SouthEast Asia is Khmer Empire (Cambodia Today) starting from famous state, Nokor Phnom Kingdom (Funan) in the A.D. first century to A.D. 6th Century and then Komvuchak or Kompuchea (Chenla and Angkor) from A.D. 6th Century to the early A.D. 15th century. That is the point for the collapse of this strong empire. The golden era of this empire is between A.D. 802 to early A.D. fifteenth century. What to brief here is about how strong Khmer Empire collapsed. After reading and researching from various sources, we can assume some criteria for the fall of Angkor Capital (Yasotharakburak Royal Capital as heart of Khmer Empire).
The main factor of the fall of Khmer Empire is the pressure from coming new power of the north, Mongol Empire. There are many kingdoms around the world collapsed by army of Mongol, except Khmer Empire. However, Mongolian army didn't overcome Khmer army, but they did create the strategy for the continuous effects to the later fall of Khmer Empire. The only thing we can see is Khmer King sent a lot of Khmer armies to fight off Mongol in the north bordering with China at that time while Mongol already occupied there. Due to difficulty of the strategic geography in the north of Khmer Empire and her strong army during the reign of King Jayavarman VII, Mongolian failed to come to the south that is Khmer Empire. Anyway, to bring the peace to the region, the later Khmer King sent ambassadors to sign peace agreement with Mongol Empire by paying a tribute in exchange for peace in the mainland South East Asia. Mongolian King demanded a lot of wealth and a jar of liver and gall bladder from Khmer people. This evidence is known with discussion by recent Khmer historian about the jar of Khmer people's liver and gall bladder as tribute happened when Zhou Da Guan came to Angkor capital to take the tributes from Khmer to Mongol King in China and a jar of Khmer resident's liver and gall bladder that was said to take two times (Zhou Da Guan didn't agree on the first jar of liver and gall bladder by giving reason that that jar of liver and gall bladder was not from Khmer people and demanded Khmer King to take the liver and gall bladder again for a new jar). This picture shows the first loss of treasure and human life from Khmer Empire to Mongol Empire. We can describe that the reason behind this is Mongolian King wanted to lower down Khmer Empire that he wasn't able to overcome as other Kingdoms around the world. In revenge for the big loss of battle field with Khmer army, Mongolian started to help Annamese (Vietnam Today) to stronger their kingdom moving towards the south that is Champa and Khmer Empire. Also, Mongolian tribes mixed with Chinese tribes together strengthened other ethnics in the north area of Khmer Empire to reinforce their local army in order to move southward and to separate small states under Khmer Empire to be independent mixing with leaders of Mongolian-Chinese ethnics including Shan and Tibeto-Burman. Mongolian army supported these ethnics to fight and break off the territory of Khmer Empire and relative Kingdoms like Mon Kingdom and Champa Kingdom as a good example. Tibeto-Burman occupied Mon Kingdom while Annamese occupied Champa and until later also occupied the southern territory of Khmer Empire (South Cambodia or Kompuchea Krom).
Now let's continue talking about the Khmer Empire. Besides the loss power on the tribute states like Mon and Champa Kingdom, Khmer Empire saw a big threat occurring within her empire is Siam (Shan who Khmer King allowed to live in the northern area of Khmer Empire as the army to protect the whole empire along with Khmer army) and Lao (Shan tribes mixed with local Khmer-Mon ethnics like Mon, Khmer, Khmu, Kuy and Lammet). In fact, Lao and Siam leaders were coming from the same ethnic group of Shan, ethnics from southern China, but most local residents of their new kingdom are Khmer, Mon, Khmu Lammet, Kuy, other Khmer-Mon ethnics and small portion are Shan ethnics.
The tribute Lao King township (King Pha Ngom who used to study in Angkor City and got married with princess of Khmer King along with 500 Khmer people specialized in arts and army) was prepared by Khmer King Jayavarman VII to create a strong battle field if Mongolian army came again, but later it became the big threat to Khmer Empire. Lao who was encouraged by newly separated Kingdom in the north with the support from Mongolian and kingship skills from Khmer Empire started to move southward and got many territories of Khmer Empire when the central power of Khmer Empire in Angkor Royal Capital (Preah Nokor Yasotharakburak) started to gradually collapsed. Lao created the new kingdom on Khmer territories called Lan Chang. Some Lao armies also occupied Mon Kingdom, Haribunjaya, and created new Kingdom known as Lanna Kingdom later gave birth to Chiang Mai Kingdom bordering with Sokhortey City (Sukhothai Kingdom). However, Chiang Mai Kingdom later was taken by Thailand at the end of 19th Century. Lao today is surely coming from the bloodline of Khmer, Khmu, Lammet and some portion of Shan tribe, Mongolian-Chinese Tribes (Tai-Kadai Ethnics).
Khmer Empire also saw other separated territories to become new independent within the empire like Sokhortey Khmer City (Later known as Sukhothai Kingdom under the Siam leader and his adopted Khmer Royal Prince, Ponhea Rong, Aka Phra Rong in Thai History), Sovannburak Khmer City (Suphanburi Kingdom or later know as Ou Thong Kingdom) and Lvothayakburak Khmer City (Lopburi and Ayuthaya Kingdom). These separated states from Khmer Empire after nearly one generation pass (100 years) of the death of King Jayavarman VII or 70 years after the coming Zhou Da Guan. With this view, we can see that Khmer Empire completely lost control on Chao Phraya basin from this period and had only stronghold in Nokor Reach Seima (Phimai or Vimai) for the northwestern area of Khmer Empire (now known as northeastern and western part of Thailand). In fact, Lvothayaburak or Lavo Kingdom and Sovannburak were still Khmer Royal related Family leading the region, but just they separated for another religion purpose that King of Angkor at that time (after Jayavarman VII) had conflict of religion between Buddhism from old reign of King Jayavarman VII and strong old religion, Hindu Religion. These separated states might want to preserve Buddhism as the bases of life with the motivation from Khmer-Siam King in Sukhothai who also sent their Khmer-Siam Royal Family to marry with Sovannburak Khmer Royal Family to control Suphanburi Kingdom and later Khmer-Siam Suphanburi Royal Family to marry with Khmer Lvothayaburak Royal Family to create new Khmer-Siam Royal Family for Kingdom in old Khmer Akyuthyea City. Later, Suphanburi Kingdom created Ou Thong Kingdom that occupied Akyuthyea Kingdom and created Ayuthaya Kingdom under the new King from Royal Khmer-Siam Royal Family line from Suphanburi. In order to give up on Khmer Empire line and adopted line from Siam of new Sukhothai Kingdom, this new king change their citizenship to Siam of all people in this new kingdom. At last, we can see two strong new kingdoms in Chao Phraya Basin were Ayuthaya Kingdom and Sukhothai Kingdom that relatively siblings separated successfully from Khmer Empire as planned and encouraged by Mongol Empire's concept. However, we can be sure that those residents in this basin are most from Khmer and Mon mixed with small portion of Shan ehtnics who used to live in Khmer Empire known as Siam and even King also coming from the bloodline of Khmer mixed with Siam. Their new Kingdom still used Khmer language in the royal communication and local people also use Khmer mixed with Siam newly created language (First created in Sukhothai by innovating on Khmer alphabet and vowels mixed with Mon and Shan language in the north area of Khmer Empire, but they still lacked of some characteristics to be the new qualified language, but they did after several sack of Khmer Capital from 15th to 16th Century for Khmer Literature). That came to the big threat for Angkor Capital that already weakened by the big loss of territories, people and tributes to support the Empire for mass construction and government. Also, King from Ayuthaya Kingdom successfully attacked Angkor Capital two times that really weakened Khmer Empire to collapse of Khmer golden age. After successfully sacked Angkor Capital of Khmer Empire, Ayuthaya Kingdom collected wealth, magical statues (now in Mandalay after Burmese army sacked of Ayuthaya Kingdom) and materials (Yant and tatoo) of Khmer Angkor, literature of Khmer along with Pali Sanskrit, arts and artists, army strategy, good ideology of Hindu religion (Devaraja and kingship), city planning documents and also fleeing hundred thousands of Khmer family to their new Kingdom. In this point, debates on Thai people today come from where should be briefly clarified that they are mostly bloodline from Khmer mixed with some portion of Mon, Lao and Shan (Siam). Another big success of Ayuthaya Kingdom was the successful sack of the strongest Long Vek Khmer Capital in the end of 16th Century with the trick of Golden Coin (Prak Duong), Siam Ayuthaya Kingdom (Preah Norensuan of Royal Family bloodline from both Ayuthaya Kingdom and Sukhthai Kingdom with warring skills under Burmese King Bayinnaung's adoption that later gained the independence for Ayuthaya Kindom from Burma and this king started to forget the bloodline of Khmer-Siam Royal Family) was so cruel at this time by destroying Khmer capital tortured and killed hundred thousands of Khmer people from Long Vek during the attack and fleeing them to Ayuthaya Kingdom. This time, Ayuthaya Kingdom also hit the jackpot for taking from Khmer Kingdom lots of treasures, Khmer Arts and artists, literature of Khmer Language and Pali Sankrit (Khmer Language Yanta which later known as Khom Alphabet on Yanta in Thailand today), Preah Ko and Preah Keo Magical Statues and hundred thousand of Khmer residents. The word, Siam, they called themselves and Khmer also called is derived from Shan commander (Shyam Guo) lived in Khmer Empire during the Golden Age of Angkor, but their real citizenship most is Khmer bloodline. Why millions of Lao in Thailand today because in the reign of King Fa Ngom went on war with neighboring country to enlarge his territories, he sent hundred thousand residents out towards the south and during the recent war, Lao also moved in another time with the permission of Siam King in 18th and 19th century along with Chinese to Thailand for their escape of the wars and to mixed with local residents who had bloodline mostly from Khmer and some of Mon (Siam King in 18th and 19th century also encouraged Mon people fleeing from Mon States to move to Lopburi and central Thailand today as well). It should be noticed that Thailand today still preserved the ideology of Devaraja and Hindu, ancient religion of Khmer-Mon (Ghost and Ancestor) mixed with Buddhism while Khmer in Cambodia nearly gives up and adopts only Hinayana Buddhism derived from Mahayana Buddhism of Khmer in the reign of King Jayavarman VII.
Another reason for the fall of Khmer Empire in the golden age of Angkor was the religion conflict that we can guess that Khmer King and Khmer Royal Family in other strong cities within Empire had broken relationship between the reign of King Jayavarman VII (Mahayana Buddhism) and King Jayavarman VIII (Hindu Region). King Jayavarman VII tried his best to recover Khmer Empire after the sack of Angkor from Champa army. He chose Buddhism with his experience to be a monk in Sri Lanka and motivated by his in-law Mon Royal Family from Preah Ko (Pegu) which their princess married him. We are not sure who was this princess because during his reign only two famous women showed in history, Indradevi and Rajadevi as the Queen. That's why in Chinese records showed that King Jayavarman VII also had powerful control over Mon Kingdom of Pegu. However, some researchers believed with the Chinese historical record that this might be the trick that King of Pegu wanted to lower the power of Khmer Empire by changing the religion from Devaraja to Buddhism and also lowering the power of Khmer King in Angkor. This might be true after the change of religion, Khmer Empire got lots of internal conflict after the reign of King Jayavarman VII who was the only one to successfully complete his project with Mahayana Buddhism. Also, this is the first time in Khmer history that Khmer King got influence from his Queen of other states for the leadership on the Khmer Empire. Due to the effect of ideology from Mahayana Buddhism on the Devaraja of Khmer King, King Jayavarman VIII quickly ordered the prohibition of Mahayana Buddhism and destroyed many Buddhist statues within the Khmer Empire, especially Preah Nokor Yasotharakburak (Angkor Capital). However, it's already late and in return it gave the opportunities and the reason for some town Kingship to separate from Khmer Empire including Sokhortey (Sukhothai firstly separated by Siam Leader who ousted Khmer Governor there and established new Siam Kingdom), and by Khmer local township governor of Sovanburak (Suphanburi) and Lvothayakburak (Lopburi and Akyuthyea) and other territories in the north of Khmer Empire (Laos today) lost gradually after that.
The last reason was the over population and mass city expansion and construction leading to the deforestation that in return causing the climate change in the region while water reservoirs might be deteriorated with the destruction of old achievement from the reign of Jayavarman VII and careless attention on those water irrigation system due to internal conflicts. The loss of residents' life on mass construction, large expenses, agricultural problems, health issue and the shortage of food for the increasing population. The big issues to the fall of Khmer Empire is the lost territories gradually along with people and tribute which usually supported the royal capital. The diseases also occurred to kill hundred thousand of Khmer people in the city. We can also notice that it might be enemy tried to destroy irrigation system leading to the weakened power of the royal city easily for later attack from the external forces.
Please further research! This is based on personal understanding from Khmer, Thai and other researchers that come together for this information!