京都所司代
When Matsudaira Sadaaki was appointed as "Kyoto Shoshidai," Tatsumi followed him to Kyoto, where he served as a liaison with other domains. It was in this role that he met many of the men who would later become famous in the Meiji period, including Katsura Kogorō, Ōkubo Toshimichi, and Saigō Takamori. Before long, though, he was transferred back to Edo and entered the Tokugawa bakufu's French-trained infantry. His French military instructors were said to have remarked, "Tatsumi is a genius of a soldier." He later served in the "Bakufu's" 3rd Infantry Regiment.