Before the thirteenth century, the region now called Thailand was composed of many small independent kingdoms. Buddhist influence had already been felt in this region as a result of contact with neighbouring countries. At the end of the thirteenth century, Theravada Buddhism gained the support of the Thai king, Ramkham-haeng. He invited Buddhist monks to teach in his capital city of Sukothai. He also established relations with Sri Lanka by sending monks there to study. Furthermore, he began the tradition of appointing a Sangha chief to oversee the administration of the monastic community. Under his patronage, all the people in his kingdom were said to have become Buddhists.
About half a century later, there was another devout and learned king called Luthai who was a strong supporter of Buddhism. He joined the Order for a period of time and is said to have initiated the Thai tradition of Buddhists becoming monks for a limited period of time, usually about three months.
In spite of the wars in the centuries following, Buddhism continued to grow in importance and enjoyed uninterrupted state support. The Thai Order is as influential today as it was in the past. However, some reforms have been introduced since the turn of the present century. There is greater emphasis on Buddhist education and the monks are actively involved not only in teaching the religion, but also in providing secular education for the lay people. Many monks still teach in the primary and secondary schools that have been set up in the Buddhist monasteries throughout the country. Many others also pursue higher studies at Buddhist colleges and universities. Over the past few decades, some monks have been sent overseas to spread the teachings of the Buddha.
1) Wikipedia: http://en.wikipedia.org/wiki/Buddhism_in_Thailand