The naval forces of the Ezo Republic were grouped around the warship Kaiten. The fleet originally consisted of eight steamships: Kaiten, Banryū, Japanese gunboatChiyoda, Chōgei, Kaiyō Maru, Kanrin Maru, Mikaho and Shinsoku. However, Kaiyō Maru and Shinsoku had been lost in a previous engagement in front of Esashi, and Kanrin Maru had been captured by Imperial forces after suffering damage in bad weather. The loss of these two major units seriously weakened the Ezo Republic side.
Imperial forces
For the operation, an Imperial Japanese Navy fleet had been rapidly constituted around the recently acquired ironclad warshipKōtetsu, which had been purchased from the United States. Other Imperial ships were Kasuga, Hiryū, Teibō,, Mōshun, which had been supplied by the domains of Saga, Chōshū and Satsuma to the newly formed Meiji government in 1868. The nascent Imperial government started with a much weaker navy than that of the Ezo Republic, both in terms of vessel strength, unity, and training. However the loss of two major units on the Ezo side previous to the main action, and most of all, the incorporation of the revolutionary Kōtetsu since April 1868 on the Imperial side, turned the tables. In addition, the Imperial government received the support of two transportation ships chartered by the United States for the transportation of its troops.
Combat
The Imperial fleet supported the deployment of troops on the island of Hokkaidō, destroyed onshore fortifications and attacked the rebel ships. On 4 May Chiyodagata was captured by Imperial forces after having been abandoned in a grounding and on 7 May Kaiten was heavily hit and put out of action. Banryū managed to sink the Imperial forces' Chiyodagata, but Banryū later sank in turn because of heavy damage. The Imperial Japanese Navy won the engagement, ultimately leading to the surrender of the Republic of Ezo at the end of May 1869. Ships of foreign navies — the British HMS Pearl and the French Coetlogon — were standing by neutrally during the conflict. The French captain Jules Brunet who had trained the rebels and helped organize their defenses, surrendered on Coetlogon on 8 June. The futureAdmiral of the fleetTōgō Heihachirō participated in the battle on the Imperial side as a young third-class officer, onboard Kasuga.