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Continuing further on to the mainland, we reach the region of Indochina. This is the name quite often given to the peninsula of Further India. Culturally, however the region is more India than China, with the big exception being Dai Viet.

When considering the region as a whole, what we can see is a general tendency towards decentralization, with previous empires in power collapsing, and emergent petty states competing to fill in the vacuum.
The fourteenth century has seen a general redrawing of the political and cultural map in the area, with the decline of the Khmer and Pagan kingdoms and the rise of several Thaic polities.

When considering the linguistic situation, the Thaic peoples have in a significant number displaced the Mon-Khmeric or Austroasiatic groups: while the Vietic and Khmer peoples maintain a degree of influence, the Mon have been effectively assimilated in the eastern parts of their homeland by the Thais; Palaungic, Khmuic , Katuic and Bahnaric peoples have also been largely displaced and pushed into the hills by the incoming Thais, who are now among the dominant power in the area. The Thais have arrived from the Nanzhao and Dali regions further northwards, and have established a number of polities and tribal groupings: the Shan and some highland tribes along between Dai Viet and Lan Xang form the major highland groupings; in the lowlands a number of Thaic polities were established, namely Lan Xang, Lan Na, Sukhothai and Ayuthaya.
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The Thaic peoples (blue) have come to dominate much of the Peninsula at the expense of the Mon-Khmeric peoples (green)
In terms of religion, Theravada Buddhism is by far the prevailing religious system in practically all of the peninsula, the notable exception being Dai Viet being of Mahayana Buddhism and a number of Hillman tribes.

Dai Viet
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The battle standard of the Tran dynasty ruling in Dai Viet in the 14th century
Dai Viet, located in the east of the Peninsula is found along the coasts of the South China Sea. The Viets, or by their autonym the Kinh are an Austroasiatic people, forming its own Vietic branch of the linguistic family, parallel to other prominent branches, as the Khmer or Monic ones. Unlike its Austroasiatic kinsmen, the Kinh had come under prolonged periods of Chinese domination, mainly during the period between the 1st century BC and the 10th century AD (with minor interruptions). As such, the area has become a genuine part of the Sinosphere – also thanks to geography, as the narrow coastal corridor along the northern borders of Dai Viet is one of the few openings through which armies can march across comfortably into the peninsula of Indochina.

During the past century, the Kinh have resisted several attempts of the Yuan dynasty to subjugate them; however they also paid tribute as a means of preventing further invasions. After the Ming dynasty has established itself in China proper, the ruling Tran dynasty of Dai Viet acknowledged them as emperors of China and agreed to pay tribute.

Fourteenth-century Dai Viet has effectively integrated gunpowder firearms into their military, gaining an advantage which has been actually the reason why they had managed to repel successive Yuan attempts at full subjugation of the country. Furthermore, the Dai Viet defenders had sought an alliance with the neighbouring Champa people to their south and used the climate and terrain to their advantage.

While Mahayana Buddhism remains by far the prevailing religion in the country, but Confucianism and Taoism are also practiced and, unlike in China, coexist with the local form of Buddhism largely in harmony. Mahayana can be seen as the effective state religion in Dai Viet during the 14th century.

With the establishment of the Ming dynasty in China, Mingjian has also established a small presence in Dai Viet, mainly in the form of Chinese merchant communities and advisors, who as a matter of fact practiced it mainly within their diasporic community, and did not try to proselytize among the wider local population..

Upper Mekong
The Upper Mekong drainage basin (1) has been held only loosely as subject mandalas or s of the Khmer Empire. The largest of these was Muang Sua (2) These areas are relatively hilly, with deep valleys and large differences between elevations at short distances. This, along with subsequent waves of migrations, has caused the area to have a striking ethnic and linguistic diversity.
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Fa Ngun, the founder of Lan Xang
Thaic peoples had been pushed southwards by the incoming Naiman invasions, and have entered the Mekong Basin.

The indigenous peoples, who used to live in the entire area are the Khmu people, related to the Kinh of Dai Viet, speaking an Austroasiatic tongue and the Katu living further southwards. The lowland regions have been occupied by the incoming Thaic peoples, who while seizing political power have also taken up the best and most fertile lowland regions along the Mekong Valley itself. These comprise of the Laotians, the Phu Thai and the Phuan peoples, practicing Theravada Buddhism. Ultimately, the uppermost highland regions are settled by the Hmong, but also various Thaic-speaking clans

By the mid-14th century, the Kingdom of Lan Xang has come to dominate this region, taking advantage of the weakening of the Khmer by both the influx of Thai peoples, and the Black Death. As a result, the invading chieftains from the north found little resistance, and were able to subdue the indigenous Khmuic peoples and force them into the higher altitudes, while keeping the fertile lowlands for themselves.

Lower Mekong
The Khmer Empire has dominated the Mekong Basin for centuries, and the Khmer have built astonishing cultural monuments in the tropical jungle environment.

However, by the 14th century, the Khmer civilization has come into decline, caused by a handful of factors. Firstly, was the worldwide disaster of the Black Death, which caused severe depopulation, especially in a humid environment such as that of Southeast Asia. With many people dead, there was not enough people to repair the irrigation system and with the collapse of the water management, an ecological breakdown might have been a likely option

However, this was not the only reason for the drastic collapse. The conversion from Vaishnavite and Shaivist Hinduism to Buddhism of the Theravada form dominant across the region has also caused some instability, as now the authority of the leaders was questioned, being no longer considered devarajas.

Thailand

The Thai tribes have managed to install themselves in the basin of the Menam , or Chao Phraya river. The Thais have established numerous realms – the older kingdom had its capital in Sukhothai, and had managed to achieve full independence in the 13th century. By the 14th century, another rival realm was established at a place known as Ayodhya to the Mon, and Ayyuthaya to the Thais, located along the lower course of the Chao Phraya River. This area had been earlier home to a number of ancient civilizations, such as the Dvaravati or the Lavo; now the Thai have established their realm in this area.
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A political map of Indochina during the late 14th century
Needless to say, the Thais, once settling down and establishing their realms – much of this was actually possible mainly to the weakening of the established polities caused by the Black Death – have naturally taken up many influences from the pre-existing civilizations and societies present in the area. Thus, it does not surprise us much that the Thais are heavily culturally influenced by the Mon people, part of whom were assimilated into the Thais, while another part maintained their old language and identity a little further westwards.
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The vast majority of Thaic peoples have settled down and became part of the mandala framework in the region.
The southernmost of the Thai realms is the kingdom of Ligor, which has often been just peripheral region, being outside of major trade routes. The kingdom is Theravada Buddhist and significantly influenced by India.

The kingdom of Lanna is located to the north of the Sukhathai kingdom, in an area that had been previously known as the Mon kingdom of Hariphuncai , a hilly region, to the north of which there were multiple Shan states.

Irawaddy Valley
The Naiman invasions into the Irrawaddy Valley have caused the collapse of the Pagan kingdom, which had previously unified the entire region. The result was that the country has broken up into a number of polities.

In the central basin of the Irrawaddy Valley, numerous at first independent kingdoms emerged, such Taungoo, Myinsaing and Sagaing. These have been united into the Ava Kingdom, which can be said to encompass most of Upper Burma. The Avans sought to reassemble the former empire. While successful in the palins, they were unable to overcome the resistance in the hills.

As you may have guessed, the dominant ethnic group of the Ava Kingdom is of course the Bamar; although interestingly, their kings were actually of Shan, that is, Thaic descent.

The Irrawaddy Delta has become the new core of the Mon, who had established their own kingdom in the area called Hanthawaddy for long or simply Pegu for short. The existence of the kingdom has been period of revival and flourishing of the Mon culture and language.
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Traditional Shan dance
In the northeast, the Shan, a group of Thai-speaking hillmen, have established numerous smaller kingdoms in the highlands, with the most powerful being Hsenwi, located in the east, and others being much smaller, like Mongyang and Hsipaw. In the uppermost Burma, the lands are populated by the Kachin, a people related to the Bamar in language, though living in a manner similar to the Shan.

To the west is a coastal strip of land, separated from the Irrawaddy Valley proper by a range of hills – the Arakan Mountains. The area between these mountains and the sea is known as Arakan and has become politically organized as the kingdom of Mrauk U. The region of Arakan is, due to geography, much closer tied to India than the Irrawaddy Valley
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Ultimately, we can see a religious map of the area​