When word of the July 8 Ikedaya Incident, in which the Shinsengumi forces had assassinated domainal samurai warriors, reached Choshu, the prudence faction, which included Masanosuke SUFU, Shinsaku TAKASUGI, and Samanosuke SHISHIDO, tried to calm the raging debate in the clan, but members of the activist faction, like the elders of the three clans, Echigo (Mototake) FUKUHARA, Uemonnosuke (Kanenobu) MASUDA, and Shinano (Chikasuke) KUNISHI, cunningly created a group designed to destroy Satsuma, against whom there was considerable enmity, and to that end they prepared for battle; Masuda and Gensui KUSAKA began gathering troops at Mt. Tenno and Mt. Takara in the town of Oyamazaki, while Kunishi and Matabe KIJIMA gathered troops at Saga Tenryu-ji temple, and Echigo FUKUHARA at the Choshu estate at Fushimi.
6月5日(旧暦)の池田屋事件で新選組に藩士を殺された変報が長州にもたらされると、慎重派の周布政之助、高杉晋作や宍戸左馬之助らは藩論の沈静化に努めるが、福原越後や益田右衛門介、国司信濃の三家老等の積極派は、討薩賊会奸を掲げて挙兵し、益田、久坂玄瑞らは大山崎町天王山、宝山に、国司、来島又兵衛らは嵯峨天龍寺に、福原越後は伏見長州屋敷に兵を集めて陣営を構える。