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Understanding of Izumo’s development during the Kofun Period (300-650 AD) has been rapidly expanding as additional kofun in the region have been discovered and explored. The construction, contents, and clustering of these tumuli provide important clues to the history of this period, a story that could previously be pieced together only through fragmentary passages in Yamato’s chronicles and Izumo Fudoki. In recent years, archaeologists have also unearthed settlements, chieftains’ residences, irrigation canals, and rice paddies, contributing to a clearer picture of social organization and material culture. The new evidence indicates that Yamato’s full domination over Izumo came later than historians once believed, and that the process by which the region was incorporated into Yamato’s confederacy was both more gradual and more complex than written sources indicate.

Between the end of the Yayoi age and the integration of the region into the Yamato confederacy during the later 6th century, chieftains in eastern Izumo formed their own confederacy, uniting the eastern plain, the mountainous Ohara to the south, and the eastern Shimane Peninsula. In the process, they advanced regional unity to a new level, with their most powerful center in the Ou basin. Although the largest tombs there are less than a quarter of the size of the great round keyholes in the Kinai and Kibi regions, they nonetheless attest to Ou’s rivalry with Yamato for cultural and political paramountcy in Izumo. Consequently, some historians now refer to Ou’s chieftains as ‘kings of Izumo’.

The story of Ou’s rise begins with a consideration of how and when eastern Izumo developed as a prominent cultural center. No bronze treasure like that of the Kojin Valley has been found in the east, but four-cornered mounds like those of the Nishi Valley were constructed in both the Ou and Jinashi basins, confirming at least two power centers in Yayoi times. In the 4th century, large square mounds reminiscent of the Kambara tomb began to appear on the Jinashi and Hakuta plains; and in the mid-5th century, one of Izumo’s earlier round keyhole mounds, Tsukuriyama Nigo Tsuka, signaled Kinai influence in the region, probably through an alliance with a local chieftain. Additional round keyholes were built nearby during the 5th century.

Interestingly, round keyholes did not continue into the 6th century. Late-5th-century tomb mounds in the Ou region indicate that the center of political gravity moved westward. According to Yamato’s historical sources, in the late 5th and early 6th centuries, Kibi’s strength was strained by feuds with Yamato kings like Yūryaku (456-479 AD) and his successors. It seems that chieftains in the Ou valley and on the eastern Shimane Peninsula took the opportunity to create an independent confederacy of their own.

Proclaiming their cultural and political independence from the Kinai, these rulers erected square or square-keyhole tombs to contrast with Yamato’s round and round-keyhole tombs. On the eastern peninsula, confederates built several tens of square-shaped tombs in seven clusters during the 5th and 6th centuries. The largest square keyhole, Kososhi Tomb near Sada Shrine, spanned about forty-three meters and was erected during the late 5th century. In the Ou basin, the thirty-meter square tomb of Oba was probably built in the late 5th century; this was followed by 6th-century tumuli at Yamashiro, Chikuya, south of the Ou River, and further east in the Jinashi and Hakuta basins. Seventeen square keyholes have been identified throughout eastern Izumo, averaging from fifty to seventy meters in length, marking the nodes of power in the confederacy. The zenith of the Ou King’s power was represented by the 92-meter Yamashiro Futago Tomb dating from the mid-6th century.

These tomb mounds were more than stately graves to glorify deceased leaders and their living descendants. Their unique shape, grave goods, and placement of such items show they were stages for ritual activity through which the dead chieftain’s spirit and authority were transmitted to his successor. Additionally, the replication of the same square shape and occurrence of similar accoutrements similarly arranged demonstrate a commonality of the ceremonial of chieftains’ burial and succession within the eastern confederacy.

Several factors contributed to the rise of this eastern confederacy, including advances in agricultural and metal technology, trade networking, and political organization. Iron-working and mastery of new irrigation techniques progressed together in 5th- and 6th-century Izumo, as cultivators needed iron tools to open the great river plains of eastern Izumo. These required the development of large-scale irrigation facilities based on advanced engineering techniques and iron implements, along with the coordination of large labor forces.

Eastern Izumo seems to have gained early access to iron goods, with iron axes dating from the mid-Yayoi period found there, although they are rare elsewhere except the Kinai. An indigenously wrought iron sword from the 3rd or 4th century, one of Japan’s oldest, was recently unearthed at a site in the Jinashi basin, and what may have been an iron-smelting furnace dating from the mid-5th century has been found at Miho on the eastern tip of the Shimane Peninsula. Ore found there was probably imported from China or Korea, indicating that trade between east Izumo and the continent continued despite efforts by Yamato and Kibi to monopolize it.

The eastern chieftains were well-placed geographically for such trade, with numerous good harbors and control over the entrance to Lake Shinji from their entrepot at Chikuya. The large square keyholes of Chikuya and Asakumi along the passageway linking the outer and inner bays witness the increasing importance of waterborne trade in the 6th century. Eastern Izumo had a number of products to sell in addition to its iron goods, including clay figures (haniwa) from kilns at Ou’s Hiradokoro and pottery from Oi, shards of which have been found as far away as Niigata.

The open expanses of alluvial basins necessitated by rice culture, the needs occasioned by trade, and competition from Yamato confederates on their borders propelled eastern chieftains toward increasing political unity and intensive governance. Confederates were bonded primarily by marital, cultural, and economic ties. Headship of the confederacy may have initially rotated among several chieftains’ lines, whose square keyholes, from forty to fifty meters in length, dot eastern Izumo. As time passed, new institutional measures established a clearer hierarchy, distinguishing superior and subordinate chieftains’ lineages. This process reached its peak by the mid-6th century when the great Yamashiro keyhole was built by a supreme Ou chieftain, an unrivaled ‘king’ of the eastern confederacy.

These tomb structures were more than just graves but stages for ritual activities demonstrating the transfer of authority from the deceased to their successors, showing a common ceremonial practice among the chieftains. The potent symbols of local deities and their sacred regalia, such as swords, tools, and mirrors, underscored the religious power these chieftains wielded. Tomb mounds continued to be built in the Ou homeland even as political alignment with Yamato strengthened over time.

Izumo’s integration into the Yamato confederacy around the mid-6th century ushered in a more stable period, diminishing the need for competitive tomb building seen previously. This period prepared the ground for the centralized imperial institutions that would emerge in later centuries, showing the blend of local and broader influences shaping regions like Izumo during Japan’s formative historical period.