One major (subsidiary) country - Ichitaikoku

...An island in the present-day Genkai Sea, the entire island is part of Iki District, Nagasaki Prefecture.
[Ancient times]
Along with Tsushima Island, it has long been considered an important maritime transportation point between Japan and Korea, and its name already appears in the Gishiwajinden as "a large (sub)state." It is said that it had more farmland than Tsushima Island and three times the population. In the pre-Taika period, it was ruled by the governor of Iki Prefecture, and under the Ritsuryo system, it was administered by the two counties of Ishida and Iki and was treated as a province, and later it was called Iki Province in both name and reality.

*Some of the terminology explanations that refer to "a major (subsidiary) country" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…現在の玄界灘上の島で，全島が長崎県壱岐郡。
【古代】
　対馬島とともに古くから日本と朝鮮間の海上交通の要衝として重視され，その名はすでに《魏志倭人伝》に〈一大(支)国〉として見え，対馬島よりも田地は広く，人口は3倍であったという。大化前代は壱岐県主に支配され，律令制下では石田･壱岐2郡を管して国に準ずる取扱いを受け，のちには名実ともに壱岐国と称された。…

※「一大（支）国」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報