Around 7th century A. D., the Buddhist tantric tradition of India quickly spread across neighboring countries like S´ri Lanka and other Southeast Asia areas like Sumatra, Java, and Malaysia peninsula; the speed of its influence was even faster than that of the S´aiva tantric tradition, which was also spreading around the same time. This was due to the conjoint result of few different conditions: the international maritime trade network, missionary zeal, Buddhist mariner communities, and the political interests of the royal family. The rapid proliferation of the Indian tantric Buddhism into Southeast Asia, however, depended heavily on the maritime trade route that had been in long-time use in business between India and Southeast Asia. This provides the answer for the question, in India or Southeast Asia, of why we have found many historical remains of Tantric Buddhism in the ancient port areas or the sites that can be reached through the rivers from the sea port.
The network of Tantric tradition established in Southeast Asia from the 7th century is well attested by the YiJing’s travelogue that appears to have been a quest for the learning of the tantra system and the collection of tantric texts. Like the Chinese monks pursuing Tantric study on the sea trade route, Indian Tantric masters took to the very same sea route to go to China in order to establish the proper lineage of tantric transmission.