The Yamato kingdom appeared on the Nara plain of central Japan between about A.D. 250 and 300 and, during the next three centuries, passed through successive stages of vigor, expansion, and disruption. Because its “great kings” (ōkimi) were buried in large mounds, these years are commonly designated the Burial Mound (kofun) period. That was when farmers converted vast tracts of virgin land into rice fields; immigrants from northeast Asia introduced advanced techniques of production from the continent; soldiers rode horses and fought with iron weapons; armies subjugated most of Japan and extended their control to neighboring regions on the Korean peninsula; and kings dispatched diplomatic missions to distant courts of Korea and China. But because no written Japanese records of that day have been preserved, and Korean and Chinese accounts do not tell us much about contemporary life on the Japanese islands, the Yamato period has long been considered a dark and puzzling stretch of prehistory.

Until the close of World War II, Japanese historians tended to think of this period as a time when the “unbroken” imperial line was mysteriously and wondrously formed. But postwar scholars have discovered new written evidence, seen historical significance in massive archaeological finds, and viewed the whole of ancient Japanese life from different angles. Egami Namio, for example, used Korean sources and the findings of archaeologists to develop the thesis that in this period Japan came to be ruled by horse-riding warriors who had invaded the islands from north Asia, and Ishimoda Shō reexamined ancient sources and found a heroic age.