Pivotal events for the Tradition: Diffusion in South-east Asia
Over the course of Buddhism’s long history, the relationship between the Buddhist community and state authority has taken many forms.
Buddhism arrived in Sri Lanka in the 3rd cen. BCE, starting a community that would have been the main outpost of Theravada Buddhism towards Southeast Asia. Missions from Sri Lanka and Northern India subsequently penetrated present day Burma (Myanmar) around 7th- 10th cen. CE, Thailand (10th-12th CE), and Cambodia (10th-12th cen. CE).  In Theravada countries Ashoka ’s image as a supporter and sponsor of the faith has traditionally been used to judge political authority.
While the Sangha and the government are two separate structures, there has been some intertwining; monks (often from elite families) have often acted as governmental advisers, and kings—at least in Thailand—have occasionally spent some time in the monastery. Moreover, Buddhist monastic institutions have often served as a link between the rural peoples and the urban elites helping to unify the various Theravada countries. Buddhist communities of these countries share the same sacred scriptures, called the Pali Canon, the oldest collection of Sutras , commentaries and monastic regulation preserved in Pali language, a Sanskrit dialect of Buddha's time.
 Such a cultural and religious bond between these countries acted as a barrier to the Muslim proselytism.
Indonesia has been an exception in respect of the neighboring countries, embracing in 7th-12th cen. CE a mixed form of Mahayana and Vajrayana Buddhism. Such a distance from the surrounding Theravada net is thought to be one of the causes why Indonesia Buddhism retreated in front of the Muslim proselytism of Muslim Traders in the 13th sec, leaving only historical marks, such as the famous Borobudur temple (see below)
Also Vietnam, due to historically relations with China, hosted mainly Mahayana

Source's analysis

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Buddhism expansion in South Asia
See above: Pivotal events for the Tradition: Diffusion in South-east Asia

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Borobodur Temple
Borobodur temple section
The monument is both a shrine to the Buddha and a place for Buddhist pilgrimage. The journey for pilgrims begins at the base of the monument, follows a path around the monument, guided by the relief expounding teachings and myths, and ascends to the top through three levels symbolic of Buddhist meanings. In fact, while Borobudur is built as a single large Stupa (see Buddhism Module I sec. 9)  When viewed from above takes the form of  a representation three "realms" of the mind: the World of Desire, the World of Forms and the Formless World.
Ordinary sentient beings live in lowest level, bound to their earthly desire. Those who have burnt out all their cravings live in the world on the level of form alone: they see forms but are not drawn to them. Finally full enlightened beings go beyond even forms and experience the formless ocean of Nirvana , which correspond to the awakening to the fundamentally Void nature of reality (basic tenet of Mahayana Buddhism), symbolized by the main dome at the center which is empty (see also Buddhism Module I p. 6).  These three sections are also thought to symbolize the feet, the body and the head of a body.
The architectural features between the three stages have metaphorical differences. For instance, relief depicting person indulging in sensual pleasure are found on the first level, and square platforms and detailed decorations illustrating mastery over desires in the World of Form disappear into plain circular platforms in the Formless World to represent the highest achievement of the Buddhist path.
Among reliefs there are series of images explaining the law of Karman Karman , others the life of the Buddha, and others again the previous lives of the Buddha.

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Thai Monks
Nearly 95% of Thailand's population is Buddhist of the Theravada school, though Buddhism in this country has become integrated with folk practices which are aimed to attract the favor of local spirits. Western observers and Western-educated Thais often draw line between these beliefs and Buddhism, but such distinction is rarely observed in more rural areas. Spiritual powers derived from the observance of Buddhist precepts and rituals are employed in appeasing local nature spirits.
Like in most other Theravada nations, Buddhism in Thailand is represented primarily by the presence of Buddhist monks, who serve as officiants on ceremonial occasions and are responsible for preserving the teachings of the Buddha.
Prior to the creation of state-run primary schools in Thailand, village temples served as the primary form of education for most Thai boys. Service in a temple as a novice was a necessary prerequisite for attaining any higher education.
Temporary ordination is the norm among Thai Buddhists. Most young men traditionally ordain for the term of a single rainy season but there are also longer ordinations in which monks are allowed to officiate at religious ceremonies in surrounding villages. After this period most young monks return to lay life, marry and begin a family. Young men in Thailand who have undergone ordination are seen as being more suitable partners for marriage; unordained men are euphemistically called 'unripe'. A period as a monk is a prerequisite for many positions of leadership within the village hierarchy.
The Thai tradition supports also laymen to undergo a period of monastic retreat, whose timeline is based on threes (three days, three weeks, three months and so on). All male Thai rich or poor are expected to do so, and often this retreat is scheduled after high school. This brings honor and good Karman to the family of the young man. Thai make allowances for men who follow this practice such as holding open a job.
Unlike in Myanmar (Burma) and Sri Lanka, the female Theravada monastic lineage was never established in Thailand.

Intercultural, interdisciplinary discussion.
Virtually every religious tradition is characterized by sacred buildings adorned with images, sculpture, engravings or architectural symbolism expounding doctrinal tenets, often for benefit of illiterates. Useful information for interesting comparison and links to Art History can be found in the following Modules' pages:
Christianity Module II sec .  1, 5
Islam Module II sec . 4, 8, 9

 

Links to other Modules
Buddhism Module I sec. 6 and 9
Christianity Module II sec.  1, 5
Islam Module II sec. 4, 8, 9