In 1056, Theravada Buddhist Monk Shin Arahan set out from Thaton to convert the King of Pagan, Anawrahta, to Theravada Buddhism, due to the king's dislike of the power and immorality Ari Buddhist monks. He was injured however, and forced to wait a year. By the time he recovered, Anawrahta had declared war on Thaton, due to his wish to combat the Khmer Empire's expansion westward. This prevented any Mon of Thaton from being able to get audience with the king of Thaton's enemy.. Thus, Shin Arahan traveled to the Pala Empire and managed to convert its king, Vigrahapala III, to Theravada Buddhism from Mahayana Buddhism.
In 1069, civil war gripped the Chola Empire. The King of Sri Lanka, Vijayabahu, saw this weakness and attacked the Chola in attempt to liberate northern Sri Lanka. He called for his fellow Theravada Buddhists to help him in this war, and with Pala's help, he was victorious. Vijayabahu wished to restore Theravada Buddhism to its once glorious state in Sri Lanka, and requested help, once more, from Pala. Vigrahapala's successor, Mahipala II, sent monks, who ordained new monks, helped rebuild temples, and taught Buddhist scriptures. Within a decade, Theravada Buddhism was restored in Sri Lanka, and by 1097, the Theravadins wished to begin proselytising out of Sri Lanka.
Knowing of the great Shin Arahan, now religious advisor to the king of Pala, Ramapala, had first attempted to convert the inhabitants of Southeast Asia to Theravada before giving up and traveling to India, they decided that Southeast Asia is an inferior place for their missionary work. The monks thus decided to travel to the various Hindu or Mayahana states of India to look for converts. They avoided the Cholas, who still despised the Theravada Sri Lankans for their defeat in the war, and instead went to the Hoysalas, in conflict with the Tamils such as the Cholas. As the Hoysala Empire was already used to religious diversity and freedom due to the large Jain population of the land, the Buddhist missionaries were allowed, and were as successful there as they were in Southeast Asia in OTL, converting many of the Kannada people and eventually managing to convert the Hoysala king in 1157, Veera Ballala II, to Theravada. He then sent missionaries to the north, spreading Buddhism further.
Meanwhile, Theravada Buddhism was spread in a more militant way by Ramapala, who conquered Assam, Orissa, and much northern Indian territory in attempt to revive his dying empire. By his death, Theravada orders were prominent throughout the area. However, when his successor Kumarapala attempted to conquer the Pagan Kingdom in 1025, the Palas suffered a grave defeat. This increased animosity in the Ari-dominated Pagan for Theravada Buddhism, leading to the Burmese of the empire attempting to stamp out the Theravada beliefs on the Mon and the Rakhine. This also led to the decline of the Pala Empire, which would lose all of its territory save for a fraction of Bengal by 1150. However, all of the successor states remained Theravada Buddhist, and with the hostile Ari Pagan Kingdom to the east, any missionaries had only one direction to travel: west, into India.
Meanwhile, missionaries continued to travel from Sri Lanka to India. The Eastern Gangas had already been converted to Theravada Buddhism by the Palas, and with the help of the inhabitants of that area, the missionaries managed to gain many converts in the areas ruled by the Kakatiya dynasty. By 1230, the Kakatiyas were majority Buddhist, and four years later their king, Mahadeva II, converted to Theravada Buddhism. From this area Buddhism continued to spread into the heart of India.
Fall of the Khmer Empire
The Khmer empire had prospered for many centuries. However, by the thirteenth century, it had entered a slow decline. The Khmer at this time ruled over the Khmer, Thais, Lao, some of the Cham, and many smaller ethnicities. The Khmer were primarily Hindu in religion, as were the Lao and Cham. An occasional Mahayana monarch every several decades did not chance their Hindu dominance, as in all cases the Hindu religion was quickly restored to be the state religion. Perhaps Theravada Buddhism could have taken Hinduism's place, but Mahayana could not. The diverse Khmer empire had become very unstable due to its extemely diverse nature.
The fall began in 1232, when the Thai city of Ratchaburi revolted from Khmer rule. The Ratchaburi kingdom managed to establish control over all former Khmer areas on the Malay Peninsula, the entire Bay of Bankok under its control, and a border with Harinpunjaya to the north. The Thais rejected the Hindu religion of the Khmer, and instead adopted the Ari Buddhism of the Burmese. This religious rejection furthered the split between the two former parts of the Khmer empire, and the two nations went to war frequently over the next century. The Khmer suffered many losses in this war, and generally fared far worse in the wars, though both nations were weakened. The Khmer became very anti-Buddhist, and the Thais were anti-Hindu. Temples of the rival religions were burned to the ground, monks were slaughtered, statues smashed, and the lay people were forced to convert. The border areas where the legal religion changed every few years were in religious chaos. The Cham took advantage of the Khmer's concentration on the Thai, and seized most of their former territory back, as well as territory as far south as the the Mekong Delta region, by 1285. The Lao revolted from the Khmer in 1299.
Thai King Phetracha the Great (r. 1280-1322) had conquered much of the Malay territory on the peninsula by the turn of the century, subjugated the Mons, and forced the Burmese into an unequal alliance. The Ratchaburi kingdom's resources were failing due to Phetracha's expansion, yet he still dreamt of war. His armies marched on the remains of the Khmer empire in 1309. His armies sacked Angor and killed the last Khmer king, Sisowath II (if the semi-propaganda stories are true, Phetracha personally was the one to kill Sisowath). Phetracha exacted a temendous tribute of the Khmer empire and returned back to Ratchaburi, using the money to pay off his massive military endeavors. Bankrupted and kingless, the Khmer empire fell into civil disarray.
If it was any consolation to the remnants of the Khmer, Ratchaburi fell soon afterwards. Phetracha's excessive spending had used up all the money taken from the Khmer, and following his death in 1322, a succession crisis destroyed the Ratchaburi Kingdom. The Santisakul dynasty was founded by King Ramesuan I (r. 1324-1342), who moved the capital to Sri Thep. The Santisakul kingdom was much more religiously tolerant, allowing the Hindus living in the western provinces to keep their religion, a religious tradition lasting until modern times. In 1356, they saved the remains of the Khmer empire as it was slowly losing territory to the Cham and Lao by vassalizing it. For the next 41 years, the Khmer survived, protected and supported by their former enemy. However, in 1397, Khmer king Thommareachea revolted against Thai rule. The Santisakul withdrew their ptoection, and the Khmer kingdom was conquered by the rising Lao nation within the year.