Together with Matsudaira Sadaaki, he left for Edo in 1861, where was permitted to attend the elite Shogunate school at Shoheiko, and where he was hailed as a prodigy.

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.

When Matsudaira Sadaaki returned, Tatsumi advocated continued military opposition to the Satchō Alliance army.

Tatsumi and the Kuwana retainers remaining in Edo followed Matsudaira Sadaaki to Echigo Province by sea, where they hoped to establish themselves in the former Shogunal territory of Kashiwazaki.

Joining up with Matsudaira Sadaaki in Kashiwazaki, he continued to fight against the new Meiji government's army, first independently, and then in concert with the Nagaoka Domain.

Tatsumi fought in the Aizu campaign, and led the Kuwana forces even north on Matsudaira Sadaaki's orders to join the Ōuetsu Reppan Dōmei, surrendering in Shōnai Domain (present-day Yamagata Prefecture) some time later.

However, he was soon pardoned by the new government, and returned to Kuwana Domain as a junior councilor after pleading his loyalty to Emperor Meiji.

From August 1886 to December 1887, he was aide-de-camp to Prince Komatsu Akihito, and accompanied him overseas on diplomatic missions to England, France, Germany and Russia, as well as the Ottoman Empire.