The period is commonly called the Tumulus, or Tomb, period from the presence of large burial mounds (kofun), its most common archaeological feature. Whereas Jōmon and Yayoi burial practices were rather primitive, from the 3rd century large tombs, both circular and uniquely keystone-shaped, began to proliferate throughout Japan, marked most especially by the enormous tumuli in and around the Ōsaka area. It is from the very construction of the tombs themselves, from an examination of the grave goods, as well as from increasingly reliable written sources both domestic and foreign that a picture of the Yamato kingdom has emerged.

In the first stage of Yamato development, tombs clustered around the Shiki area of Yamato province (modern Nara prefecture), in the southwestern corner of the Nara (Yamato) Basin. Rulers there held sway over an expanding portion of the archipelago. The Yamato kings (called kimi and written with the appropriated Chinese characters for “great ruler”) were centered around Mount Miwa, the object of worship. Although the kimi exercised both secular and sacred functions, it seems that their primary focus was a priestly one, based on a sacred connection with Mount Miwa. Archaeological findings suggest, however, that improved agricultural techniques—such as the use of iron tools for cultivation and improved techniques for leveling and flooding paddy fields—allowed the Yamato rulers to exercise control over significant manpower resources, both to construct large tombs and to expand the area under their control outward from the Nara plain.

From about 350, power shifted north to the Saki area, near the present city of Nara. The nature of the burial goods in the tombs constructed there, the legendary accounts in Kojiki and Nihon shoki, and records from the continent all indicate that this was a period of Yamato expansion throughout the archipelago and even into the Korean peninsula, where, as mentioned above, its armies were engaged in the warfare between the three Korean kingdoms on the peninsula. Although the rulers continued to worship Mount Miwa, the religious focus of the court seems to have been concentrated upon the Isonokami Shrine at Tenri, south of Nara. The rulers there seem to have been somewhat more military in nature than their Miwa predecessors, and archaeological findings suggest that the most treasured items of the Isonokami Shrine were in fact weapons—especially the so-called “seven-pronged sword” (shichishitō), which now is designated a National Treasure.

Thus, by the end of the 4th century, Yamato was a kingdom well settled on the Nara plain with considerable control over the peoples of the archipelago. It was in contact with Chinese rulers, exchanged diplomatic envoys with several of the kingdoms on the Korean peninsula, and was even strong enough to have sent an army against the powerful state of Koguryŏ, which then dominated the peninsula. Yamato was most closely associated with the southwestern kingdom of Paekche, whence came the “seven-pronged sword.” Contact with the mainland, although involving conflict, also encouraged a marked rise in standards of living in the archipelago, as many of the fruits of advanced Chinese civilization reached Japan via people from the peninsula. Weavers, smiths, and irrigation experts migrated to Japan, and the Chinese ideographic script also was introduced at that time, together with Confucian works written in this script. Claims by historians prior to World War II that Paekche paid “tribute” to Japan and that Japan conquered the southern tip of the peninsula where it established a “colony” called Mimana have since been largely discounted by historians in both Japan and Korea.

The Yamato court reached its peak in the early 5th century, during the second stage of its existence. Once again, there was a shift in the center of power, this time directly westward to the provinces of Kawachi and Izumi (modern Ōsaka urban prefecture). The 5th century was one of spectacular development for Yamato, as evidenced by the enormous keyhole-shaped tombs in the suburbs of the modern Ōsaka region. The move into this region is thought to have resulted in a power shift either between or within clan federations. It is now customary to regard the 5th-century rulers as a new line, distinct from those of the Shiki and Saki areas.

What distinguishes the 5th-century tombs from earlier ones is both their enormous size—the tomb attributed to the semilegendary emperor Ōjin is some 1,380 feet (420 meters) in length—and their character. These rulers had access to great power in order to construct their tombs. It has been estimated that the construction of Ōjin’s tomb would have taken 1,000 laborers, working from morning to night, four years to complete. The goods associated with these tombs are far more military in nature than those found in the earlier tombs: iron swords, arrowheads, and tools; armor; and all the trappings of a mounted warrior culture. All this suggests that the 5th-century rulers represent a more military, secular line of leaders in comparison with the priestly kings of the earlier Yamato area.

While most historians regard the 5th-century rulers as representing a new line, there is disagreement over their origin. Some have postulated an invasion of continental “horse riders” who seized control in the archipelago and established a new line of rulers. Myths related to Ōjin’s birth on the Korean peninsula while his mother was supposedly leading Yamato armies there, the location of the center of power at the port of Naniwa (modern Ōsaka), Ōjin’s arrival there by boat, and the awesome size of the tombs (which suggest excess slave labor available for their construction)—all these hint tantalizingly at a conquest theory. The consensus, however, still supports an indigenous shift in leaders relying on control of increased agricultural output and monopolizing superior military technology. From the court at Yamato, its rulers extended control along the Inland Sea and beyond, developing more sophisticated offices and units to control the peoples of the archipelago.

The Yamato polity

The pattern of administrative control established is called the uji-kabane system. Uji is usually translated as “clan” in English. The uji are thought to be extensions of original agricultural communities, perhaps what early Chinese records referred to as “states.” Essentially, farming communities were associated into lineal groups, united by the belief that harvests would be bountiful if proper respect was paid to the group’s ancestral deity (kami). Heads of the community functioned primarily as priests, mediating the relationship between the group and its deity. As clans joined together—probably largely by conquest—vertical relationships began to develop between heads of the communities and the queen or king at emergent courts. By the 5th century, these groups, possibly already called uji, were drawn together into economic, military, religious and familial ties with the Yamato kings. Some scholars have even argued that uji were purely political units, so designated by the Yamato ruler. Uji appeared first in the Nara Basin, in close association with the court; as the Yamato kingdom developed greater power, uji appeared in other areas as well.

By the 5th century, the Yamato ruler was designating the heads of the most powerful uji, who developed close ties with the ruler over time. The Yamato court was thus headed by a hereditary ruler, while its members were drawn from the group of powerful clan leaders awarded kabane (titles). The two major titles appear to have been muraji and omi, held only by clan leaders of powerful communities serving in the area of the Yamato court. Lower-ranking titles were awarded to leaders of smaller, distant clans who nonetheless swore allegiance. The highest officers of the emerging state were the ō-muraji and the ō-omi, the heads and representatives of those two groups.

Another factor that aided the expansion of the emergent state was the economic and military support of occupational groups, called be or tomo, attached to the court and its supporting uji. Structurally somewhat similar to clans, these occupational groups were distinguished by providing a special service to the court or to a superior clan. Earlier be were more likely to provide personal services or specialize in religious functions, but as the power of the Yamato court spread throughout the archipelago in the 5th century, newer be came to be involved with the production of weapons, armor, and mirrors or with the construction of irrigation systems. Many of them were composed of recent migrants from Paekche who specialized in raising horses or ironworking; in fact, the term be itself is of Korean origin. Some be were directly controlled by the court, including special ones called nashiro and koshiro set up for the support of certain royal relatives. Others were controlled by powerful clans directly in the service of the court, such as the yugei, the quiver bearers, who were attached to the Ōtomo clan, a major military support group for the Yamato ruling house.

Yamato relations with Korean states

If the 5th century represents an expansion of power throughout the archipelago, it also was a time of involvement in Korean affairs as the struggle for peninsular hegemony intensified. At the time of Yamato’s expedition against Koguryŏ in the late 4th century, Paekche and Yamato found themselves allied against Silla or Koguryŏ (or both); while the latter looked to northern Chinese kingdoms for support and legitimation, Yamato and Paekche usually turned to southern China. In fact, Yamato dispatched some 10 embassies to the Nan (Southern) Song between 421 and 478.

Paekche was frequently attacked by Koguryŏ during the century, prompting continued requests for assistance from Yamato; it is recorded that Paekche even sent a crown prince to Yamato as a hostage on one occasion and sent the mother of the king on another. Yet, probably because of internal dissension, Yamato did not dispatch any troops to the peninsula, although a lengthy memorial sent with the embassy of 478 and presented to the Nan Song emperor requested that the Yamato king Yūryaku be appointed commander of a large army being raised for dispatch against Koguryŏ.

Yamato’s interest in Korea was apparently a desire for access to improved continental technology and resources, especially iron, which was especially plentiful near the lower reaches of the Naktong River in the south. Yamato apparently gained a modicum of power in this region, controlled by the league of the Kaya (Japanese: Mimana) states between Paekche and Silla, though the exact relationship—whether ally or tributary—is unclear. But in the 6th century, Silla became militarily powerful, and Yamato faced several reversals in the area, ultimately being driven entirely from the peninsula when Silla annexed the Kaya league in 562.