After some cooperative work, such as when Satsuma purchased weapons for the Choshu clan, who were officially regarded as a rebels and could not purchase them, both signed the secretive agreement to form an alliance at the Satsuma clan's residence in the presence of Takayoshi KIDO and Saigo in Kyoto in January 1866.
逆賊となり表向き武器の購入が不可能となっていた長州藩に変わって薩摩が武器を購入するなどの経済的な連携を経た後、慶応2年（1866年）正月、京都薩摩藩邸内で木戸孝允・西郷らが立ち会い、薩長同盟の密約が締結された。