On the 21st day of the first month of 1866, the rival domains of Satsuma and Chōshū concluded an alliance in Kyoto. Komatsu Tatewaki and Saigō Takamori represented Satsuma. Kido Takayoshi represented Chōshū. Japanese reference works name Sakamoto Ryōma and Nakaoka Shintarō of Tosa as the men who mediated between the two domains.
The cautious Kido then asked Ryōma to confirm the terms in writing. Ryōma wrote his endorsement in red ink on the back of Kido’s letter.
In 2018, the historian Machida Akihiro argued that Ryōma was not the mediator. In his view, Komatsu and Kido had settled the terms a few days earlier, and Ryōma came as a witness.