On March 7, 1866 (March 8 according to another theory), an alliance between the Satsuma and Choshu clans, which Shinsaku had been promoting with Kogoro KATSURA, Kaoru INOUE and Shunsuke ITO, was concluded at the residence of the Satsuma clan in Kyoto, with Ryoma SAKAMOTO, Shintaro NAKAOKA and Hisamoto HIJIKATA from the Tosa clan acting as intermediaries.
さらに慶応2年（1866年）1月21日（一説には1月22日）、土佐藩の坂本龍馬・中岡慎太郎・土方久元を仲介として、晋作も桂小五郎・井上馨・伊藤俊輔たちと共に進めていた薩長同盟が京都薩摩藩邸で結ばれる。