Tanzawa Mountains
Aikō District was one of the ancient subdivisions of Sagami Province, extending from central Sagami north to the border of Musashi Province between the Sagami River and the Tanzawa Mountains. The district offices were located in what is now part of Atsugi. The area was part of a vast "shōen" controlled by the Ōe clan, and their descendants, the Mōri clan of Chōshū from the Heian period through the Sengoku period. It later became a contested area between the later Hōjō clan of Odawara and the Takeda clan of Kai. In the Edo period, it was nominally part of Odawara Domain, although large portions were "tenryō" territory controlled by the "shōgun" in Edo through various "hatamoto". From 1706, a branch of the Ōkubo clan in Odawara was permitted to establish the Ogino-Yamanaka Domain on a portion of the district. Other portions of the district came under control of Karasuyama Domain of Shimōsa Province.