In the 13th century, wandering missionaries from the Mon-Khmer-speaking parts of Siam, Burma, Cambodia, and Sri Lanka played an important part in this process.
When Prince Tamalinda returned after ten years of ordination, he was a Thera, a senior monk, capable of administering ordination into this vigorous Theravada lineage, which insisted on orthodoxy and rejected Mahayana "innovations" such as tantric practices.