When the first millennium started, small countries were existing side by side in Japan and there was no central political power. A huge new design tumulus (front-square and rear-rounded) Hashihaka was built in the mid-third century in Yamato and this heralded a new era. From around this time onward Yamato was gradually growing its power and gaining its hegemony over the country. This period is called “Kofun (tumulus) Period". The size and design of burial mounds were controlled by Yamato and they symbolized the power of the people buried and their relations to the central power in Yamato.
By the middle of the 6th century, the political power of Yamato became so overwhelming that it did not need to build big burial mounds to display its political power. Therefore, Hashihaka type huge tumulus almost disappeared in the area around Yamato. On the other hand, in the far remote Kanto region, front-square-rear-rounded type tumulus were continuously built until the beginning of the 7th century, and in addition various types of haniwa (terracotta figures) were developed and used in mass. It is thought that Hashihaka type tumulus design was originally used for Yamato to infiltrate its power into every corner of the country, but at this stage it was used by the Kanto lords to show their power locally.
In the ancient Kanto region, civilization penetrated from the advanced western region, on the one hand along the Pacific coast into the inland sea such as Tokyo Bay and further into rivers such as the Old-Tone River, and on the other hand through the inland roads from Nagano into Gunma and Tochigi area. Kanto region was thus divided into coastal area and inland area.
The Shiba Maruyama Kofun, which still remains in Tokyo, is thought to date from the early Kofun Period (4th century) and to have been built by a powerful family at a base along the sea route of Tokyo Bay. However, there are few large burial mounds that remain on the west coast of Tokyo Bay, and it seems that the eastern coast of Tokyo Bay dominated the route in Tokyo Bay, and many burial mounds remain to date from Ichihara to Kisarazu and Futtsu in Chiba Prefecture.
In the late Kofun Period, from the 6th century to the beginning of the 7th century, large tumuli straddle only in the inland area, namely Kamitsu-Kenu (currently Gunma prefecture), Shimotsu-Kenu (present Tochigi prefecture) and north-Musashi (Saitama prefecture). In the coastal areas, the influence of Yamato came directly from early on and powerful local clans disappeared, while in the inland areas, the indigenous rulers would have been powerful until later.
The use of Chinese character had not yet spread over the country in the Kofun Period and this is the major obstacle to reaching correct understanding of the history during the Kofun Period. It is almost impossible to get first-hand evidences of historical events with the help of archeology, because we hardly excavate any character engraved into durable materials such as stone or metal.
In Japan, two historical books, Kojiki and Nihon Shoki, happened to be compiled in the 8th century at a relatively early stage of history, but their articles might not correctly reflect genuine facts. However, it is not easy to support what is written there with the contemporary evidences. For example, although there are some 160,000 ancient tumuli in Japan as a whole, unlike China, there is no grave magazine in the burial area and scarcely anything is written in burial chambers and it is almost impossible even to identify the person buried in the grave. Very few stone monuments were built in that time.
As far as after the end of the Kofun period is concerned, the situation is a bit better. A considerable number of wooden tablets were recently discovered in the soil from the time of Emperor Tenmu in the latter half of the 7th century to the Nara period in the 8th century and I am looking forward to the research results.
Under such circumstances, it is very noteworthy that 115 gold inlaid characters were engraved on the iron sword excavated from the Inariyama Kofun in Gyoda City, Saitama Prefecture, and that there was the word "Wakatakeru" indicating the Emperor Yuryaku. Similarly, a sword with the words "Wakatakeru" was found in Kumamoto Prefecture, demonstrating that the rule of the central government during the time of Emperor Yuryaku at the end of the 5th century extended from Kyushu to Kanto.
Regarding the Inariyama iron sword, it is the common understanding among experts that the dead person buried in the place where the sword was found is NOT "Owake" who was the head of guards of Emperor Yuryaku and made the sword. The first reason is that lords coming from such a local area as Musashi were not in a position to command the emperor's guards, pushing aside the central lords.
Inariyama Kofun is a majestic front-square-rear-rounded burial mound with a length of 120 meters, which is suitable for a tomb of a great lord. However, the burying place where the sword was found is not in the center of the mound, but on the surface of tumulus. Therefore, the man who was buried there must not be the lord of the tumulus, but maybe someone from the same clan. This is the second reason.
It is therefore possible that, the iron sword made by "Owake" was, for some reason, given to a man from Musashi when he served at Yamato and the sword was buried with him. We should also note that the letters on the iron sword clearly state that it was made in 471. However, the sword is thought to have been buried about 30 years later from that date, at the beginning of the 6th century and after the end of the rule of Emperor Yuryaku.
The front square part of the Inariyama Kofun was destroyed and only the rear part remains, but now the front part has also been restored. Information about the burial mound's owner is still asleep deep in the burial mound, as the depths of the burial mound have not been investigated.
The Inariyama Kofun is believed to be the oldest of the nine burial mounds preserved and maintained as the Sakitama Kofun cluster, and was built at the end of the 5th century. The second oldest tomb is the Maruhakayama Kofun, which is 105 meters in diameter and is the largest circular tomb in Japan. Of the tumuli, this is the only tumulus which is not the front-square-rear-rounded. The third oldest burial mound, the Futagoyama burial mound, which is thought to have been built in the first half of the 6th century, has a length of 132 meters. What kind of drama was unfolded involving the lords of these burial mounds?
According to the Nihon Shoki, in the first half of the 6th century, during the reign of Emperor Ankan, there was a dispute between Kasahara no Atai Omi and Oki from the same clan in Musashi, and Oki asked Kamitsu-Kenu no Kimi Oguma to help him. It is also reported in Nihon Shoki that Omi asked the Yamato court for help and won the battle. It is almost sure that Sakitama burial mounds belong to the clan of Musashi’ s Kunino-Miyatsuko (Kunino-Miyatsuko is a title given by Yamato to the powerful lord of a region), and the owner or the Futagoyama burial mound, which is the largest burial mound among front-square-rea-rounded type tumuli, might be the winner of the clan's battle.
When the Yamato administration dispatched troops to the Korean Peninsula, Musashi and Keno's powerful clans joined the operation and brought back various advanced cultural artifacts from the Peninsula. Together with them many people came from the Peninsula to the region and promoted the development of the region. Many of the artifacts excavated from the Sakitama Kofun cluster show a direct connection to the Peninsula.
The Kanto Region was also designated by the imperial court as the migration destination for people from Kaya, Baekje, Silla and Goguryeo who fled the turmoil of the Korean Peninsula. Among the people who moved to Musashi, those from Goguryeo have maintained their cohesiveness centered on the royal family. There still exists Koma Shrine at the foot of the mountain in Chichibu, where the descendants of the Goguryeo royal family serve as priests till today.