Ihai is a policy under which the Ritsuryo-seifu Government (the ancient Japanese government of centralized governance) (Imperial court) forced Emishi (northerners), who lived in present-day Tohoku area, to move to Kanto region and to the west of Honshu, Shikoku and Kyushu from the 8th to the 9th century.
After the middle of the 7th century, Yamato Dynasty tried to subordinate Emishi. However, as the ritsuryo system (a system of centralized government based on the ritsuryo code) was established, a 'small empire' came to be established with a kind of Sino-centrism, which distinguished people in inland Japan under the Emperor's rule from Tang as a neighbor country, Silla and Balhae as foreign countries, and Emishi as barbarians as well as Hayato (ancient tribe in Kyushu) looking upon them as social outcasts. The following view given by Eiichi ISHIGAMI and other historians has been supported to a certain extent: under the circumstances, hastening to establish Japan as a nation by bringing all the people and lands in Japan under the Emperor's rule, although Emishi, Hayato and other Japanese were the same Wajin (people who lived in the Wa country [Japan]), the government concealed the reality that it was not yet able to control these frontier people, and making the best of a difficult situation, established a 'small empire' to include all people who were currently under the Emperor's control. Because of this, Emishi and Hayato were thought to be barbarians by the 'inland' people; they were despised and excluded by the people under the Emperor's rule. On the other hand, Mutsu Province and Echigo Province (later the northern part was separated off as Dewa Province) became unstable as a result of this policy, which led in turn to the Emishi policy being unsettled all the time.
Under the pretext of fighting social outcasts the Ritsuryo-seifu Government used military power to conquer Emishi and expand its power to the north. As a result, the government had many Emishi (residents and captives) under their control. They were called Fushu (subjected barbarians). Though they followed Ritsuryo-seifu Government, it is possible that they had contacts with other Emishi, called Ifu, who were still excluded as barbarians. Accordingly, in 725, the government decided to move 130 such subjected barbarians to Iyo (Shikoku) and 578 to Chikushi (Kyushu) and 15 to Izumi (article on February 25, 725 in "Shoku Nihongi" [Chronicle of Japan Continued]). This is the first record of Ihai. Following that, in 738, 115 subjected barbarians were sent to Settsu Province (Osaka Prefecture today) via Suruga Province (Shizuoka Prefecture today), which was confirmed in the official tax record of Suruga Province. It was confirmed that the guards for Sakimori (soldiers deployed for border defense) escorted this group of subjected barbarians, which suggests that they were sent to Chikushi (Kyushu) in the end, where Sakimori were stationed, to engage in similar work to the Sakimori.
However, from around the time when the so called "38 years' War" began in 774, the Ihai policy rapidly intensified. In addition, even barbarians classified as Ifu were moved, and they were sent further afield across the nation, including Kanto area. There are some theories about why this was, such as the ones below, but no conclusion has been reached. One theory is that due to the great efforts by SAKANOUE no Tamuramaro, major opposing powers within Emishi were removed, which produced many subordinate groups, the rapid increase of which was a threat to order in certain places. Another theory is that Mutsu Province and Dewa Province were unable to afford the Fushu allowance anymore which provided for the re-education of the barbarians. A third theory is that the government's aim during this war changed to controlling the Ou area for the sake of the stability of the nation, and there were attempts to bring Fushu under the Emperor's rule. A fourth theory is the opposite of the previous theory: after conquering the Ou area, in order to maintain the 'small empire' as before special residential areas were created for the outcasts. In 813 an imperial order was issued, under which provincial governors in ryoseikoku (provinces) took on the additional responsibility of managing subjected barbarians, making them civilized, and protecting and nourishing them; the following year another imperial order was issued, making the Emishi the Emperor's people. Also, the government set forth the idea of using Emishi in place of Sakimori and the army (of ancient Japan) which had become weak (Daijokanpu [official documents from Daijokan to local governments] on 14 January, 859, "Ruiju sandai kaku [Assorted regulations from Three Reigns]"). In fact, however, because barbarians did not adapt themselves to the places they were sent, and had conflicts with and rose against the locals quite often, a petition from Mutsu Province to keep and control barbarians in their original place was created in 811, and was accepted. After this petition was accepted, Ihai was no longer carried out on a large scale; instead, the barbarians who had already been sent to other places were sent again to new places in small groups. In 897, a policy was announced to send Emishi back to the Ou region. According to this policy, most Emishi who had been sent throughout the nation went back to the Ou region and lived there again.