In 1578 Oda Nobunaga called on his retainers to attack the Mōri clan. Nagaharu almost decided to lead the Oda troops, but after hearing that the low-born general Hashiba Hideyoshi, whom he did not respect, was allied with the Oda faction he revolted, instead allying himself with Hatano Hideharu of Tanba province.
This led to Nagaharu being besieged by Hideyoshi's troops on the orders of Nobunaga. Nagaharu took a stand in Miki Castle, starting the Siege of Miki. The siege did not go well for Hideyoshi, and with a revolt by Araki Murashige and the help of the Mōri clan Nagaharu successfully repelled the Oda force. But Hideyoshi returned and this time instead of launching a direct assault, he launched multiple sieges against smaller castles like Kamiyoshi Castle and Sigata Castle to cut off the support from Mōri. This led to a rapid depletion of food, and in 1580, with no hope of another reinforcement from Mōri clan, Nagaharu committed seppuku in exchange for the lives of the troops in Miki Castle.
- (Japanese) Japanese Wiki article on Bessho (20 Sept. 2007)
- Matsubayashi Yasuaki 松林靖明 (1996). Bessho ki: kenkyū to shiryō 别所記: 研究と資料. Osaka: Izumi Shoin 和泉書院.