Yamato Japan and the Gaya Confederacy, which were located directly across the Korea Strait from one another, had very close diplomatic and commercial ties since prehistory until the conquest of Gaya by Silla in 562 AD.
Until recently Japanese historians had believed that Gaya, once called Mimana in Japanese, was controlled by Japan for most of its existence as an overseas colonial possession. Though this theory has been largely refuted since the 1970s, it remains a sensitive and re-occurring issue in modern-day Korea–Japan relations.
Gaya and Japan
Around 300 BC the introduction into Japan of agriculture and metallurgy from the regions of Korea which would later become Gaya helped bring a transition from the hunter-gatherer Jōmon culture into the Yayoi culture, though it is unclear whether this transition occurred due to a large-scale or small-scale invasion by a mainland Asian group or by the adoption of imports by the native inhabitants of the Japanese islands. As the nations of Yayoi Japan gradually unified, Gaya was the first foreign country with which they established relations and by the beginning of the Kofun period around 250 AD Gaya remained the country with which Japan had the closest links. Many Japanese archaeological sites dating from the Yayoi and the subsequent Kofun period testify to these contacts, with one holding artifacts that were 70% of Gayan origin.
Commerce and migration
A thriving bilateral trade existed between Gaya and Japan. Gaya exported pottery, horse tack, and most crucially, iron and iron products, to Japan, and among the items Japan sent back to Gaya were bronze implements and jade magatama produced at Itoigawa. For most of its existence Gaya enjoyed a monopoly on major trade routes both between Japan and China and between Japan and the rest of Korea, and was thus an important intermediary between Japan and other Asian countries.
Due to its proximity to China the Gaya Confederacy was also politically and technologically ahead of Japan and played an important role in Japan's development as a nation. Mass migrations of Gayans to Japan, often to escape strife between Gaya and neighboring Korean states, allowed Japan to produce on its own some of the products it had previously needed to import. Japan's sue pottery style and its first iron production were founded by Gayan immigrants, and Gayan warriors and officials were widely employed by the Japanese government.
Military alliance
In 399 AD Japan formed a military alliance with Gaya and Baekje in opposition to the growing power of Goguryeo and Silla and from then on the Japanese often dispatched military forces to protect Gaya's territorial integrity, possibly in exchange for iron or other goods. The defeat of one such Japanese expedition to Gaya is described on the Gwanggaeto Stele constructed in 414 AD. Japanese mercenaries were also allowed to serve in Korea, and the characteristically Japanese keyhole-shaped burial mounds which started to appear in Gaya and Paekche at the end of the 5th century are believed to be the tombs of those who died in Korea. While the unity of the Gaya Confederacy was falling apart under pressure from neighboring states, one of the Gayan states, Ara Gaya, even formed a joint military mission with Japan in 540 AD which was staffed by Japanese officers.
In 554 AD Japanese soldiers fought alongside the armies of Gaya and Paekche at the Battle of Kwansan Fortress. The defeat of this alliance by Silla sealed the fate of Gaya which was conquered by Silla in 562 AD. By this time Paekche had already broken Gaya's monopoly on trade with Japan and would replace Gaya as Japan's most important conduit for foreign trade and mainland Asian culture.