The recent discovery of Sueki unglazed ceramics at an archaeological site in Uji, Kyoto Prefecture, has experts rethinking the chronology of early exchanges between Japan and the Korean Peninsula.

The discovery revealed that the production of Sueki wares began in Japan in the late fourth century, 20 to 30 years earlier than archaeologists had believed, indicating that people from the Korean Peninsula who produced the ceramics arrived in Japan around the same time.

Horseback riding and iron weaponry were also introduced from the peninsula around the same period, bringing a wave of technical innovation to the country.

The ceramics were excavated from the archaeological site along with a cypress board that was dated back to 389 through dendrochronology, the science of dating events through the comparative study of growth rings in trees and aged wood.

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One version of founding of Yamato:

Prince Homuda, an influential member of a Paekche royal family and a military leader close to King Kun Ch'ogo, had sometime earlier given thought to finding new territory where he could carry on the government of his kingdom in peace. He spoke with other members of the Paekche royal family about his plans to conquer an eastern land, "a fair land encircled [on] all sides by blue mountains." The Imperial Princes, who had similar ideas, agreed. King Kun Ch'ogo chose Prince Homuda to lead the Imperial Princes and a naval force on an expedition against the mysterious land to the East. After rebuilding his command with choice Paekche troops and with the blessings of King Kun Ch'ogo and a "solemn declaration of alliance," Prince Homuda marched east toward the Kaya territory, located a "port of passage" in the southernmost Kaya state of Imna, and set sail for a New World.
In the winter of 369 AD, Prince Homuda's expeditionary force landed on the northern shore of Kyushu at Hakata Bay on the westernmost of Japan's large islands. On the rich agricultural plain near the present site of Fukuoka, the new arrivals from Paekche established a foothold and began building settlements. They spent the next three years repairing and refitting ships, making weapons, training, storing provisions and getting ready to subdue the territory. Prince Homuda's army pushed eastward for six years, encountering fierce resistance from many of the clans in its path. After subduing its opposition by surrender,outright conquest, or death, the expeditionary force finally halted on the rich agricultural plain formed by the Yodo and Yamato Rivers at the head of Osaka Bay. Having gained control over the central part of the country, Prince Homuda proclaimed the creation of his new kingdom, taking its name from the surrounding region and giving the country its first official "name" - Yamato.
Prince Homuda, founder of the Imperial Clan, was enthroned in 390 AD, with an imperial title befitting his stature - King of Yamato. Until the mid-7th century, the word "Yamato" was written as "Wa" (a term used by Chinese historians), but read as "Yamato." The great flowering of Japanese history began on the Yamato Plain and in the area of the Nara Basin, where the emergence of the Yamato Kingdom set a foundation for future Japanese civilizations. As late as the 3rd century AD, there were no horses in Japan. The formal arrival of horses in Japan occurred in 404 AD, when the King of Paekche sent A-chik-ki with a stallion and a mare as tribute. A-chik-ki cared for the two horses in stables on the slopes of Karu, a site that became known as Mumaya-saka (Stable Hill). Thus, the horse eventually bred by the Japanese was not a local horse, but a breed introduced into Japan from Korea.
A nearly continuous flow of people from Paekche and surrounding kingdoms followed in the wake of Prince Homuda's expedition, sailing the Tsushima Strait to Hakata Bay. From around 400 AD, the Yamato Plain and Nara Basin were settled predominantly by immigrants from Paekche, as royal families, generals and their descendants staked their claims in the new land by building palaces and capitals. Many of these clans gained sufficient economic and military power to control to enjoy a hegemony over the surrounding aristocracies that made them both wealthy and powerful. During this period, the Yayoi culture dissolved into the newer Yamato culture as political alliances and outright conquest gradually brought about a loose-knit unity that coalesced most of western Japan into a nation he Register of Families. Prior to the establishment of the first capital at Nara in 710 AD, most Yamato sovereigns lived in palaces built in or near the present-day village of Asuka, about 25 km to the south. At the time, the Asuka region was the political and cultural center of Japan, home to most of the Yamato kings who ruled before the 8th century. Unlike the situation in Korea, Yamato had no permanent capital city. When a king died, his successor usually transferred the capitol (site of his palace) to a new site in the Asuka region. Most of these palaces were, like temple shrines, fairly simple structures that could be built without much effort.
The Korean and Chinese clans that established roots in Japan were far more sophisticated than the native Yayoi culture and had knowledge, skills and technologies associated with a more advanced civilization. The Yayoi faced a markedly militaristic people with an appetite, if not fondness for warfare. Although the Yayoi jealously clung to the belief they were descendants of native gods, their primitive society underwent a radical change as it absorbed and adapted itself to a higher type of culture he Evidence of the Tombs.
The cultural linkage among China, Korea and Japan produced a considerable amount of interchange among the Chinese of the Lolang Commandery, the people of Paekche and the Kaya Federation, and the inhabitants of Kyushu and islands in the Tsushima Strait. Envoys from Paekche visited Yamato as early as 367 AD and diplomatic embassies between Paekche and Yamato continued almost every year up to the death of King Kun Ch'ogo in 375 AD. His son and successor, Kun Kusu, continued this close relationship with Yamato Japan, cultivating friendships with Jin China on the one hand and Yamato Japan on the other. Because of the natural, intimate relationship between Paekche and Yamato, successive Yamato rulers maintained and administered a port facility in the Imna area (Mimana in Japanese) at the southern tip of the peninsula to serve as a direct short-cut crossing route to Japan. Kaya became a stopping off point for the many Yamato trade missions that traveled between Japan and the Lolang Commandery along the Taedong River. The Yamato who lived in Imna were neither colonists, nor conquerors, but agents who operated by permission from the King of Kaya, who restricted them to the immediate area of the port.

Historical and geographical circumstances have always put Japan on the periphery of major cultural centers, where it became primarily a recipient or borrower of cultural and technological innovations. Nevertheless, throughout its history Japan has always had the unique ability to work out stylistic refinements of borrowed technology and culture to suit itself. With the emergence of Yamato Japan, yet another major player stepped on to the stage, an ambitious, quick study that was ready, willing and able to take act out its destiny in the drama of Korean history.

http://www.koreanhistoryproject.org/Ket/C02/E0206.htm