The Tenbun Rebellion occurred from Tenbun 3 (1534) to Tenbun 5 (1536) and was an internal rebellion in the Ōsaki clan of Mutsu Province. This event is separate from the Tenbun Conflict – a series of clashes continuing from Tenbun 11 to Tenbun 17 (1542 to 1548) between supporters of Date Tanemune (a sengoku daimyō and the fourteenth head of the Date clan) and those of his eldest son, Date Harumune.
In the sixth month of 1534, Ōsaki Yoshinao became upset when a retainer named Niida Yoritō refused to serve and headed out to eliminate him. However, the Ujiie clan who served as the shitsuji, or minister, for the Ōsaki and members of branches of the Ōsaki, including Furukawa Mochihiro and Kōsen Naokata, backed Yoritō so Yoshinao fought against them as well. The original reason(s) for Yoritō to refuse to serve are unclear so the basis for these individuals to support Yoritō so far as to betray their lord are also uncertain. According to one account, the Ujiie clan and others deployed on the pretext of confirming the goodwill of close relatives.
The retainers who remained with the Ōsaki clan were concentrated in the Naruse River Basin while those in the rebel army were primarily in the Eai River Basin, posing a serious confrontation among those in subordinate relationships to the Ōsaki clan.
In the end, attempts to suppress Yoritō failed and the conflict spread across the territory of the Ōsaki. Concerned at these developments, Yoshinao headed out to ask for reinforcements from Date Tanemune, based at Koori-Nishiyama Castle in Mutsu Province. The negotiations continued for an extended period so, although Yoshinao temporarily returned home, he was primarily absent for a two-year period. During this time, the rebel army was in a superior position but, within the Furukawa clan, an elder named Yonetani Hiromasa scorned his lord, Mochihiro, and insisted he would return to the service of Yoshinao whereupon he betrayed his lord and was killed. Within the Ujiie clan, a rebellion occurred and their main base at Iwatesawa Castle was temporarily occupied by the rebel army.
In the fifth month of 1536, Date Tanemune finally deployed and, in the sixth month, the allied forces of the Ōsaki and Date surrounded the main base of Furukawa Mochihiro at Furukawa Castle. On 6/19, an attack on the castle began and, on 6/21, Mochihiro was killed and the castle toppled. By the tenth month, the Kōsen and Ujiie clans were forced to surrender and the internal conflict subsided.
Although the conflict within the Ōsaki clan had at once subsided, in fact, in 1539 and in 1541, conflicts erupted and, as a result, the Ōsaki clan received reinforcements in exchange for accepting the son of Date Tanemune (Ōsaki Yoshinobu) as an adopted child. Next, a dispute arose within the Date clan (known as the Tenbun Conflict) that resulted in the downfall of Tanemune from his position as head of the clan and the killing of Yoshinobu. Yoshinao’s natural son, Ōsaki Yoshitaka, succeeded Yoshinao as head of the family. The Ujiie clan who served as the minister of the Ōsaki served as intermediaries to forge a new relationship with the Date, but, in a case of irony, Yoshitaka came into conflict with his minister, Ujiie Yoshitsugu, in regard to Niida Osabe Yoshikage who appeared to be a member of the same family as Niida Yoritō. Date Masamune assisted Yoshitsugu and thereby intervened in the affairs of the Ōsaki clan, leading to the Battle of Ōsaki.