Translator's Note: This is an unauthorized translation of an article originally published in Yomiuri Shimbun on January 12 2011. Photographs and the author's notes have been redacted but the original page numbers have been retained for use in citations.
Soldiers of Ancient Japan Dispatched at Korea's Request?
Research overturns the theory of "territorial advance"
In the middle of the Kofun Period (at the end of the 4th century and into the 5th century) when massive keyhole-shaped tumuli were being constructed, the armies of ancient Japan crossed over the sea into Korea. The story behind this is gradually being revealed through the findings of new Japanese and Korean excavations. In the past this was understood as being an invasion of Korea, but Japanese and Korean researchers are currently of the view that they were soldiers and mercenaries called upon by Korea.
The account in the Nihon Shoki, which states that Japan had established a colony called Mimana in the south of the Korean peninsula, has been called into question by research conducted since the 1970's, but the Gwanggaeto Stele erected in Goguryeo in 414 AD leaves little doubt that troops from Japan entered the peninsula. Nevertheless, within Korean academia even the fact that Japanese troops crossed into Korea had traditionally been denied as being an attempt to revive the Mimana theory.
However it turns out that about 20 suits of armor which are believed to have been made in Japan have been discovered in recent years even in the hilltop forts of Baekje as well as in Gaya including in the ruins of Mangi Castle. The chances that Japanese troops participated in actual fighting has risen. "Most are burial accessories, but they have also been found apart from tombs and that has great significance", says Shinsaku Tanaka, curator at Ikeda City Museum of Osaka Prefecture who specializes in the military archeology of the Kofun Period.
Exactly what kind of people travelled overseas to Korea? Tanaka thinks that a group of military commanders who returned from the fighting in Korea were buried in six tumuli in the eastern part of the Sakurazuka grave cluster in Toyonaka, Osaka Prefecture. The cluster is composed of circular and keyhole-shaped burial mounds, including the 55-meter-long Oshishizuka Tumuli, and while their dimensions are not particularly large, a total of 28 suits of armor and helmets were unearthed there. This figure is comparable with the number found in the Furuichi grave cluster and the Mozu grave cluster, both in Osaka, which form the core of huge tumuli over 400 meters in length.
Tanaka theorizes that "The ones who actually crossed into Korea were not the powerful nobles established since the 3rd century in Yamatai, they were the emerging powers in Japanese society like those interred in the Oshishizuka Tumuli" and furthermore "It wasn't inspired by territorial ambitions, these were soldiers dispatched at the request of the Korean side."
The major reason for this request lies in the state of affairs in east Asia at that time. In 316 AD the Western Jin Dynasty that had united China collapsed and with that rival removed from its northern front Koguryo expanded southward down the peninsula. It could be that Baekje and Gaya which were under pressure from Goguryeo asked Japan for support.
Takehiko Matsugi, professor of archeology at Okayama University, thinks the Japanese wanted iron in return. He says, "At that time Japan had not consolidated its unification under the emperors and the powers of each region had their own autonomy and right to conduct diplomacy. I suppose they were like the wako pirates of later times who engaged in both war and trade."
At the end of the 5th century more than 10 keyhole-shaped tumuli, which had existed only in Japan, suddenly make their appearance in the south of Baekje. Park Cheun Soo, professor of archeology at Korea's Kyungpook National University, believes that the owners of these graves are the leaders of the Japanese mercenaries employed by the king of Baekje. At that time Japan was in chaos over the imperial succession and Park thinks that forces from Kyushu were hired by Baekje. In addition, in 2008 a wooden coffin made in Baekje and decorated with gold and silver as well as gilt bronze horse tack was dug up from the Yongduri Tumulus, a keyhole-shaped burial mound located in the southwestern tip of the Korean peninsula. However, Park argues that "Both of these things are believed to have been bestowed by the king of Baekje and they corroborate the mercenary theory."
It appears that Japan's military ties with the Korean peninsula had a great impact on the process through which ancient Japan grew into a unified nation.