The assassination of Ii Naosuke, the chief minister (tairō) of Japan, took place outside the Sakurada Gate of Edo Castle, marking a significant moment in the turbulent period leading up to the Meiji Restoration. Naosuke, a prominent figure in the Tokugawa shogunate, was instrumental in negotiating treaties with Western powers, which were met with fierce opposition from nationalist factions, including many rōnin and other samurai loyal to the shōgunate's traditional power structure. On March 24, 1860, a group of angry rōnin, led by a samurai named Nakaie Masazane, ambushed the minister as he approached the castle on foot. The attack was executed with precision, resulting in Naosuke's death from multiple stab wounds. His assassination was not only a personal vendetta but also symbolic of the broader societal struggle occurring within Japan as it faced external pressures and internal discord.

In the aftermath of the assassination, the political landscape in Japan shifted dramatically. The shogunate was weakened, and the incident prompted a crackdown against rōnin and other dissenters. The assassination also provoked a series of retaliatory actions and violent conflicts, further accelerating Japan's transformation from a feudal society to a modern state. Naosuke's death ultimately highlighted the division between factions advocating for modernization and those wishing to uphold the traditional samurai values, laying the groundwork for future conflicts leading up to the Meiji Restoration in 1868.