In the late
Nara period, after the establishment of a centralized government under the
Ritsuryō system, the
imperial court sent a number of military expeditions to what is now the
Tōhoku region of northern Japan to bring the local
Emishi tribes under its control. The Emishi were able to successfully resist the Japanese for several decades; however, in 802 AD, the
Chinjufu-shōgun Sakanoue no Tamuramaro led an expedition with 4000 troops from the ten provinces of eastern Japan (
Suruga,
Kai,
Sagami,
Musashi,
Kazusa,
Shimōsa,
Hitachi,
Shinano,
Kōzuke and
Shimotsuke) and built Isawa Castle as his stronghold within Emishi territory in the valley of the
Kitakami River. With the defeat of the Emishi chieftain
Aterui, many of the Emishi tribes in the
Shiwa District submitted to Japanese rule. In 803 AD,
Shiwa Castle was established in what is now part of the city of
Morioka to serve as an administrative center of the imperial government. However, the site was prone to flooding, and in 811 AD, and Shiwa Castle was abandoned in favor of
Tokutan Castle approximately ten kilometers to the south. The garrison at Isawa Castle was reduced to 700 men in 815 AD, of whom many were local Emishi (known as ) in the service of the imperial court. By the middle of the 9th century, mention of Isawa Castle largely disappears from the historical record, although it reappears as a stronghold of
Abe no Munetō in the
Zenkunen War against the forces central government led by
Minamoto no Yoriyoshi. It may have been destroyed by the
Gosannen War of 1083. ==Overview==