Sri Lanka played a crucial role in the development of the Theravada version of Buddhism in Southeast Asia. It has become the predominant religion in Myanmar since the 11th century, where it was reformed in the Sinhalese tradition in the 14th century. In Thailand, conversion to Theravada Buddhism was a means to demonstrate independence from Khmer rule in the 13th century, when the first Thai kingdoms were established. Sri Lankan monks were invited to the then Thai capital, Sukothai. However, the Sri Lankan version of Buddhism, imparted by Thai monks, became prevalent in the Khmer kingdom, too, though slightly later on. Since the 14th century, Theravada has been the national religion of Cambodia. That's the period after the demise of Angkor. During the Angkor period, there had already been some contacts between monks from Sri Lanka and Cambodia. Furthermore, the island of Lanka, as the location of the decisive battle in the sacred Ramayana epic, has become the setting of the most important national Khmer epic, which like in many other literatures of Aoutheast Asia, a version of the said Indian epic. Lanka as a mythical place had already been represented in Khmer art from the Angkor period, most famously at the immensely large carving in the western gallery of Angkor Wat from the mid 12th century. Rama's fight against Ravana, the demon king of Lanka, can also be seen in several lintel carvings from the late 12th cebtury Bayon period, which was mainly Buddhist (Mahayanist, not Theravadic). The photo of such a relief depicting the battle of Lanka is taken from the newly published Preah Khan page, by courtesy of Mr. Sopheak.
Nuwan Chinthaka Gajanayaka,