I assume its both sides of the Boshin war that the opening post talks about
After two centuries of seclusion, Japan was forced to open it's doors to foreign powers for trade when Commodore Matthew Perry arrived in Edo in 1853. Against gun-boats and modern military weapons, the Japanese had no chance in a war and were forced to bend to the wills of the Western nations. This lead to economic turmoil and high tensions between locals and foreigners, and most of the blame was put on the ruler of Japan at the time, the Shogun.
In particular, samurai in Choshu and Satsuma really hated this and decided that the Shogunate should be abolished and power should be returned to the Emperor. The Shogunate initially resisted, but after the death of the Shogun Tokugawa Iemochi in 1866 and Emperor Komei in 1867, the new Shogun, Yoshinobu, tried to reorganize the government in a way where the Tokugawa family would still have sway in politics and keeps their lands, but the Emperor would be in control. He quickly resigned his powers to the emperor, but Satsuma and Choshu were not satisfied, and demanded that the Shogunate be dissolved and all lands stripped away from the Tokugawa family.
Initially agreeing, Yoshinobu changed his mind and refused to heed to the demands of Choshu-Satsuma alliance, who by now had moved into to occupy Kyoto, the site of the imperial palace. He decided to attack the combined forces, thus beginning the Boshin War.