In 838, a fleet of four ships departed from Japan and sailed to China. Approximately 650 people were on board, among whom were diplomats, sailors, artisans, and Buddhist monks. They were going to present tributary gifts from the Japanese court to the Chinese emperor and expected to receive return gifts of even greater value. Moreover, the members of the tributary delegation also intended to learn advanced technologies and knowledge and to acquire desirable continental goods and useful books from China.

Between the seventh and early ninth centuries, diplomatic delegations like those formed the main channel for exchanges between China and Japan. The Japanese court dispatched tributary delegations to China, also known as kentōshi 遣唐使 or qiantangshi, at an approximately fifteen-year interval. The 838 delegation, however, was the last Japanese embassy to Tang China and the diplomatic relationship between China and Japan thereafter fell into a long suspension: the next official tribute mission occurred in 1403 after a six-century long hiatus. Chinese dynastic histories and Japanese official documents maintained records of the diplomatic delegations but were much less comprehensive in recording the unofficial exchanges. Therefore, to investigate Sino-Japanese relations during the hiatus of the tributary relationship, exploring sources beyond the official records is necessary.

This chapter takes the mid-ninth century as an example and shows how Buddhist archives serve as a crucial source for studying Sino-Japanese relations in the absence of official records.