This chance came in 1588, when Mogami Yoshiaki and his forces were supporting Date Masamune on the far side of the Ōu Mountains. Honjō forces invaded and quickly overran Oguni Castle and went on to capture all of the Shōnai area. However, in 1598, the Uesugi clan was relocated on order of Toyotomi Hideyoshi from Echigo to Aizu, leaving the Shōnai area as a detached and vulnerable holding of the Uesugi. The defenses of Oguni Castle were again strengthened. In 1600, as part of the wider Sekigahara campaign, the forces of Uesugi Kagekatsu attacked pro-Tokugawa Mogami Yoshiaki. Possession of the Shōnai area enabled the Uesugi to attack the Mogami in a pincher maneuver and the Uesugi forces captured much of the Yamagata Basin from the Mogami: however, the victory of Tokugawa Ieyasu at the Battle of Sekigahara caused the Uesugi to abandon their territorial gains in order to defend against an expected attack. The Mogami quickly recaptured the Shōnai area and Oguni Castle once again became a border castle.