From Empire of the Winds: The Global Role of Asia's Great Archipelago, by Philip Bowring (I.B. Tauris, 2018), Kindle pp. 92-93:
Local rulers imported Indian practices as a means of strengthening their own power. They used kingship systems allied to priests and religious practices to formalize power structures, and founded dynasties from what had been often transient political entities reliant on the strength of individuals. There was no sudden transition, and it was probably not until around 400 CE that Indian cultural influence was well advanced beyond the ports, and states were using an Indian calendar and adding Indian words to local vocabulary. Indian cultural and political ideas also spread gradually up the Mekong through the Tonle Sap lake to much of modern Cambodia and Thailand’s Chao Phraya valley. Indian influence was stronger on the peninsula because of the greater presence of Indian merchants who conducted the westward trade, while those from Nusantaria took charge from there eastward. By about 300 CE Indian cultural influence reached the Vietnam coast, where the Cham states emerged as an Indianized evolution of the Sa Huynh culture.
For the whole of the first millennium CE Indian cultural and economic influence on the maritime zone, as on most of mainland South East Asia, was predominant. China itself had little direct impact. Given the later regional role of China and particularly of the Chinese diaspora, this may seem puzzling. The reasons included China’s late development of ocean-going vessels; the complexity of its writing system; the possible lack of portability of the Confucian system compared with Hindu and Buddhist teachings; the northern, landward orientation of Chinese strategic thinking; and the lack of experience with foreign trade compared in particular with the merchants of coastal India and the sailors of Nusantaria. Transport of Chinese goods was a catalyst for trade, but not by the Chinese.
Quite how far Indians themselves were the leading force in Indianization is a matter for conjecture. Writing in the mid-20th century, Indian historian R.C. Majumdar argued that the region was effectively colonized by a large number of Indians – Brahmans, merchants and others who established ruling groups throughout the region, supplanting indigenous leaders and bringing a complete culture and kingship system to the backward Austronesian and Khmer entities. French historian of the region Georges Coedès proposed a milder version of the Majumdar thesis, suggesting Brahmans were prime movers whether by imposing their rule, allying with local rulers for mutual benefit or marrying into the local elite.
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The shortage of evidence leads history books to telescope time when, in reality, the process of Indianization was a gradual one over several hundred years and from both north and south India. Early Indian influence appears mainly to have come from northern India via Bengal. But links with the Tamil south, and with Buddhist Sri Lanka grew in importance. There is evidence from Tamil sources of extensive trade with Java as early as the second century CE. A Tamil inscription from the third century CE has also been found in Thailand, which indicates that a Tamil merchant had settled there. This was a time when many Tamils, like the Sinhalese, followed Buddhism. The Pallawa script of southern India became the basis of many others, including Lao, Khmer, Javanese and Balinese. Its rounded form is said to be especially suited to palm leaves, the main writing material used in India and most of South East Asia until around the seventeenth century CE.
Indian culture may have come later to Java than to the peninsula and mainland, with Hindu beliefs preceding Buddhist teaching. Brahmanism was introduced first but then Buddhism. The slow spread of Buddhism was indicated by a report from the fifth century CE by Chinese Buddhist pilgrim Fa Xian, who found that on Yeh Po Ti (Java): ‘various forms of error and Brahmanism are flourishing while the faith of Buddha is not worth speaking of.’