Indians were never such people to stay at home. They have been moving out from ancient time to different parts of the world for trade and other activities. As far as the Indian contact with Southeast Asia is concerned, it appears to be as old as fifth century BC.
Jatakas the Buddhist texts belonging to this period refer to Indians visiting Suvarnadvipa (island of gold), which is identified with Java. Such early contacts with Southeast Asia are confirmed by the recent archeological finds of pearls and ornaments of agate and carnelian, the semi-precious stones of Indian origin, from the coastal sites in Thailand, Vietnam, Indonesia, etc. These finds belong to as far back as first century BC.
According to the Chinese traditions, the first kingdom in South east Asia was founded at Funan (Cambodia) in the fourth century AD by a brahman known as Kaundinya who had come from India and had married the local princess. However, Indian and South-east Asian contacts became closer from 5th century AD onwards when inscriptions in Sanskrit language start appearing in many areas. It reached its peak during AD 800 - AD 1300 when many kings and dynasties with Indian names emerge all over Southeast Asia.
The Southeast contact was largely on account of trade. Southeast Asia is rich in cardamom, sandal wood, camphor, cloves, etc. which formed important items of trade between India and the West. Initially, the Indian traders appear to have settled along the coast, but gradually they shifted their network to the interior.
Along with the traders came the priests particularly the Buddhist and brahmanas, to meet the ritual requirements of the Indian settlers. It created a situation for the spread of Indian social and cultural ideas in South east Asia. But Indian contact did not uproot the local culture. It was rather a case of peaceful intermixing of Indian concepts with local cultural features.
Therefore, for example, while Sanskrit was accepted as a language of court and religion in Southeast Asia the regional languages continued to be used side by side, and we find many inscriptions in mixed Sanskrit and local language. Similarly, the concept of varna was known to the south east Asians and brahmanas were respected in society, but social divisions were not rigid as it was in India.
The most important empire which come to be founded in South east Asia in the 8th Century AD was the Shailendra empire. It comprised Java, Sumatra, Malay-Pennisula and other parts of the Southeast Asian region. They were a leading naval power and on account of their geographical position controlled the trade between China and India as well as other countries in the west.
The Shailendra kings were followers of Buddhism and had close contact with the Indian rulers. One of the kings of this empire, built a monastery at Nalanda in the ninth century, and at his request the Pala king Devapala of Bengal granted five villages for its upkeep. Similarly in the eleventh century another king was permitted by the Chola king Rajaraja I to build a Buddhist monastery at Nagapattam on the Tamil Coast. The Shailendras also built a beautiful temple dedicated to Buddha at Barabudur in Java. It is situated on the top of a hillock and consists of nine gradually receding terraces.
Besides Buddhism, the worship of Hindu gods such as Vishnu and Siva was also quite popular in southeast Asia. The temples dedicated to them have been found at various places. They show distinct traces of Indian influence and inspiration. One of the most famous temples, dedicated to Vishnu, is Angkorvat temple built in the 12th century by Surya Varman II, the king of Kambuja (Cambodia). It is surrounded by a moat, filled with water. It has a huge gopuram (gateway) and number of galleries, the walls of which are decorated with sculptures based on themes drawn from Mahabharat and Ramayana.