The Tenbun Conflict was a series of clashes continuing from Tenbun 11 to Tenbun 17 (1542 to 1548) between supporters of Date Tanemune (the fourteenth head of the Date clan) and those of his eldest son, Date Harumune. This is a separate event from the Tenbun Rebellion, or Tenbun no nairan, which was an internal conflict in the Ōsaki clan of Mutsu Province from 1534 to 1536.
In 1514, Tanemune expanded his influence in the region by sending numerous daughters to neighboring families for marriage or adoption. Over a period of thirty years as head of the clan, he took control of ten districts, became the military governor of Mutsu Province, and, at the onset of the Tenbun era (1532 to 1555), brought under his governance influential daimyō families in southern Oushū, including the Mogami, the Sōma, the Ashina, the Ōsaki, and the Kasai.
Prelude to the conflict
Upon swiftly amassing the largest territory in Oushū, Tanemune aimed to strengthen the governance of the Date family by drafting a set of provincial laws, ledgers, and the like promoting a further consolidation of power. Tanemune found his son-in-law, Sōma Akitane, to be particularly helpful in these activities, so he desired to return a portion of the former territory of the Sōma, namely, the Uda and Namegata districts, to the clan, but Tanemune’s eldest son, Date Harumune, vigorously opposed the plan.
In addition, Tanemune sought to further expand the influence of the family by sending his third son, Tokimunemaru (later known as Date Sanemoto) for adoption by Uesugi Sadazane, the military governor of Echigo Province. This caused a decisive rift in his relations with Harumune. Meanwhile, in 1540, those opposed to this plan in Echigo, including Honjō Fusanaga of the Agakita Group (a group of fiercely independent families based in the northern portion of Echigo), raised arms that escalated into conflict. Tanemune aimed to intervene militarily in an effort to oppose them.
Initially, Nagao Tamekage, the prior deputy military governor who continued to wield influence in Echigo, supported the plan to bind the families through the adoption of Sanemoto, but, after learning of Tanemune’s intentions to intervene militarily, made clear his opposition to the plan. Following the death of Tamekage in 1541, this perspective did not change under his successor to the clan, Nagao Harukage, his eldest son and deputy military governor of Echigo. Harukage then learned that, owing to frustration over the absence of a successor, Sadazane intended to enter the priesthood and lead a secluded life, upon which he was obliged to inform Tanemune of his consent to the plan. Nevertheless, a series of internal conflicts arose in Echigo and opposition to the plan persisted. As the deputy military governors, the Nagao were justified in supporting the plans of the Uesugi as the military governors of Echigo, but after the death of Sadazane, the Nagao were preoccupied with the risk of a break in relations with the Uesugi, and on guard to possible intervention or invasion by the Date into Echigo.
In an effort to resist those opposed to his plan, Tanemune selected 100 elite mounted soldiers to accompany Tokimunemaru on his move to Echigo. Out of concern that dispatching the most capable soldiers would weaken the clan, Harumune garnered support from others including Nakano Munetoki, Koori Kagenaga, and Makino Muneoki who all opposed Tanemune’s policies to consolidate power and, in 1542, finally decided to imprison his father.
The request from the faction in Echigo who supported the plan for Tokimunemaru to become the designated heir of Uesugi Sadazane may have originated from Nagao Tamekage after he sought to improve relations with Sadazane following internal conflicts, and, in so doing, initiated negotiations with Tanemune brokered by Nakajō Fujisuke. However, while Tamekage requested Sadazane as the military governor to honor his wishes, Tanemune desired through the adoption of Sanemoto to have Echigo fall under the command of the Date clan. This would lead to the removal of the Nagao clan as the de facto holders of power in the province. Owing to direct military intervention by Tanemune against the faction in Echigo opposed to the adoption and the acquisition by Tamekage of an edict from Emperor Gonara to eliminate Tanemune, negotiations temporarily broke down and both sides of the dispute fell into disarray.
Details of the conflict
In the sixth month of 1542, Harumune launched an assault against Tanemune while Tanemune was returning from a falconry outing, apprehended him, and then incarcerated him at Harumune’s base in Koori-Nishiyama Castle. Tanemune, however, was rescued by a close associate, Koyanagawa Munetomo, and escaped to Kakeda Castle, the base of his son-in-law, Kakeda Toshimune.
Beginning with Sōma Akitane, Tanemune called for support from assorted daimyō with whom he had blood ties, so the internal conflict in the Date clan at once enveloped other daimyō families in Oushū in a major conflict. In the early stages, those on the side of Tanemune held an advantage with the support of assorted daimyō. In Mutsu, forces led by Ōsaki Yoshinobu and Kurukawa Kageuji advanced to the Shibata District and pinned down Rusu Kagemune. In Dewa, Ayukai Moritsugu, Kamikōriyama Tameie, and Mogami Yoshimori suppressed most of the Nagai District. However, in 1547, discord arose between Tamura Takaaki and Ashina Moriuji on the side of Tanemune, and after the two began to fight, the Ashina switched to the side of Harumune. At once, this changed the course of the conflict in favor of Harumune, triggering a succession of those abandoning Tanemune. In the ninth month of 1548, through the mediation of Ashikaga Yoshiteru, the shōgun, a settlement was reached on the condition that Tanemune would retire and transfer control of the clan to Harumune, bringing to an end the conflict.
The personal directive from Yoshiteru that served as the basis of the mediation was not drafted in the Kōji era when Harumune served as the Master of the Eastern Capital Office, so troubles between father and son may have in fact persisted after the end of the Tenbun Conflict. Meanwhile, battles broke out between the factions in Echigo who were for and against the plan for Tokimunemaru to become the designated successor to the Uesugi clan. Advocates of the adoption led by Uesugi Sadazane and Nakajō Fujisuke lost to the faction who opposed the adoption led by Nagao Harukage (the deputy military governor) and the Agakita group so that the plan for Tokimunemaru’s succession came to a final end.
Consequences of the conflict
Owing to the ongoing conflict over a six-year period, the influence of the Date clan that had steadily increased during the time that Tanemune served as its leader experienced a rapid decline. First, daimyō families in the Ouu region including the Ashina, the Sōma, the Mogami who had been subordinate to the Date clan took advantage of the conflict to assert their independence and expand their influence. In particular, the Ashina clan grew in stature as an influential daimyō on a par with the Date. Moreover, in the case of both the Ōsaki and Kasai, sons sent by Tanemune for adoption (Yoshinobu and Harukiyo) died in the conflict, so Tanemune’s efforts at taking over these clans failed.
Within the Date family, Kakeda Toshimune and others who supported Tanemune disagreed with the peace proposal and continued their resistance against Harumune. This required over five more years to suppress. Furthermore, during the conflict, a senior retainer of Harumune named Nakano Munetoki expanded his influence by sending his son, Hisanaka, to become the successor to the Makino clan. After becoming the most powerful figure in the family, Munetoki began to asset his authority. After the end of the conflict, Harumune conducted investigations and judicial proceedings of family members with the intention of further consolidating his power, causing his senior retainers to insist on rights to exclude military governors and associated officials from entering designated areas. Consequently, the after-effects of the conflict did not subside until the era of Harumune’s successor, Terumune.
Meanwhile, in Echigo, Uesugi Sadazane died in 1550 without having arranged for a successor, bringing to an end the Uesugi clan as the military governors of the province. This paved the way for the deputy military governor, Nagao Kagetora (the younger brother of Nagao Harukage, later known as Uesugi Kenshin) to become the lord of Echigo whereupon he gained the status of a sengoku daimyō both in form and in substance.