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Kōji (弘治) was a Japanese era name (年号, nengō, "year name") after Tenbun and before Eiroku. This period spanned the years from October 1555 through February 1558.^[1] Reigning emperors were Go-Nara-tennō (後奈良天皇) and Ōgimachi-tennō (正親町天皇).^[2]

• 1555 Kōji gannen (弘治元年): The era name was changed to mark an event or a number of events. The old era ended and a new once commenced in Tenbun 24.

The name originates from the following Chinese passage: 「祇承宝命、志弘治体」.

• 1555 (Kōji 1, 1st month): A border war began between Mōri Motonari, daimyō of Aki Province, and Sue Harukata, daimyō of Suō Province.^[3]
• 1555 (Kōji 1, 11th month): The Mōri forces surrounded the Sue defenders in the Battle of Itsukushima. When the outcome of the battle became clear, Sue Harutaka committed suicide; and others, including Odomo-no Yoshinaga, followed Harutaka in suicide. This victory, and the subsequent consolidation of the Mōri holdings were owing to Motonari's four sons: Mōri Takamoto, Kikkawa Motoharu, Hoda Motokiyo, and Kobayakawa Takakage.^[3]
• 1555 (Kōji 1): The forces of Takeda Shingen and Uesugi Kenshin met at the confluence of the Saigawa and the Chikumagawa in Shinano Province; and the fighting was known as the Battles of Kawanakajima.^[4]
• 1556 (Kōji 2): The Ōmori silver mine fell into the control of the Mōri clan during a campaign in Iwami Province.^[5]
• September 27, 1557 (Kōji 3, 5th day of the 9th month): Emperor Go-Nara died at age 62.^[3]

• Nussbaum, Louis Frédéric and Käthe Roth. (2005). Japan Encyclopedia. Cambridge: Harvard University Press. ISBN 978-0-674-01753-5; OCLC 48943301
• Sansom, George Bailey. (1958). A History of Japan, ISBN 9780804705257; ISBN 9780804705240; OCLC 16859819
• Titsingh, Isaac. (1834). Nihon Ōdai Ichiran; ou, Annales des empereurs du Japon. Paris: Royal Asiatic Society, Oriental Translation Fund of Great Britain and Ireland. OCLC 5850691

• National Diet Library, "The Japanese Calendar" -- historical overview plus illustrative images from library's collection