Tenbun War

The Tenbun War (Japanese: 天文の乱) was a civil conflict within the Date clan between the clan head Date Tanemune (伊達稙宗) and his eldest surviving son Harumune (晴宗) that took place over six years, from the 11th to the 17th year of Tenbun (1542 to 1548), during the Sengoku period of Japan, and the series of wars that broke out across the Tōhoku region as a result. It is also called the Utsuro War (洞の乱).

Background

Date Tanemune, who inherited the family headship in the 11th year of Eishō (1514) and became the 14th head of the Date clan, spent the following thirty years expanding his power by sending many of his children to neighboring daimyō (through adoption and marriage). As a result, in the early years of Tenbun he controlled ten districts of Mutsu Province, obtained the post of shugo of Mutsu, and brought most of the southern daimyō of Ōu¹, including the Mogami clan, Soma clan, Ashina clan, Ōsaki clan, and Kasai clan, under the subordination of the Date clan.

Having established a dominant position in Ōu, Tanemune strongly pursued centralization, including enacting the Bunkoku-hō² to control the Date clan, which had expanded greatly in a short time. At the same time, he adopted a conciliatory policy toward the daimyō under his control and attempted to grant territory to his son-in-law Soma Akitane (相馬顕胤). However, Tanemune's eldest surviving son Harumune strongly opposed this grant, and conflict between father and son began to emerge.

Afterward, an event occurred that decisively deepened this conflict between father and son. Tanemune, seeking to further expand the power of the Date clan, attempted to send his third son, Tokimune-maru (later (伊達実元)), to become the adopted heir of (上杉定実), the of . However, in the 9th year of Tenbun (1540), lords in Echigo, including (本庄房長), who opposed the adoption, rose in revolt, and the dispute developed into open conflict. To counter these opponents, Tanemune decided to have 100 retainers selected to accompany Tokimune-maru when he entered Echigo. However, Harumune feared that sending elite retainers and soldiers would weaken the Date clan, and with the support of senior vassals who opposed Tanemune's centralization policies, such as (中野宗時) and (桑折景長), he resolved to place his father under house arrest.