薩長同盟
The outcome of the Anglo-Satsuma War of 1863, along with the Richardson Affair and the September 1863 coup d'état in Kyoto, convinced Ōkubo that the "tōbaku" movement was doomed. In 1866, Ōkubo met with Saigō Takamori and Chōshū Domain's Kido Takayoshi to form the secret Satchō Alliance to overthrow the Tokugawa through the mediation of Sakamoto Ryōma of Tosa Domain.