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,[1][2] 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.
[5][6][7] 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.
[10][23] The Nihon Shoki, an ancient text of Japanese history, states that Empress Jingū led a military expedition on the Korean peninsula which defeated the forces of Gaya, Paekche, and Silla, and made Gaya, which was referred to as Mimana,[24][25] into a colony under the control of a Japanese administrative office called "Mimana Nihon Fu".
[26][28] However, the theory continued to be supported well after 1945, including in the works of such influential Western Japanologists as George Sansom, John Whitney Hall, and Edwin Reischauer.
[34] Historians have also called into question the historical reliability of the Nihon Shoki's account of events, as Japan was far behind Gaya technologically and still politically fractious at the time when Empress Jingu was alleged to have successfully invaded Korea.
[35][36][37] In 1963 North Korean historian Kim Sŏk-hyŏng became the first scholar to call into question the existence of a Japanese colony in ancient South Korea.
He instead put forward the so-called "branch kingdom theory", stating that "Mimana" was not in Korea at all but was a settlement of Gayan immigrants on the Japanese mainland in modern-day Okayama Prefecture which was eventually absorbed into Yamato Japan.
[38] Though Kim's theory is today not widely accepted outside of North Korea, it did spur other historians to pursue further research in this field.
[39] The theory which is today dominant among both Korean and Japanese historians is the "diplomatic delegation theory", originally put forward by Masayuki Ukeda in 1974, which proposes that "Mimana Nihon Fu" as described in the Nihon Shoki was actually a sort of Japanese embassy handling diplomatic relations between the independent states of Gaya and Japan.
However, the controversy expanded to other issues, including the fact that the textbooks still discussed ancient Japan's conquest and rule of Mimana.