The assassination took place on 24 March 1860, on Double Third Festival where all daimyos stationed in Edo (modern Tokyo) were scheduled to enter Edo Castle for meetings.
The assassins attacked Ii's entourage just outside the Castle, near the Sakuradamon (Sakurada Gate) when Ii was reaching the premises.
Ii had been warned about his safety, and many encouraged him to retire from office, but he refused, replying that "My own safety is nothing when I see the danger threatening the future of the country".

Ii's entourage was composed of around 60 samurai guards and Ii's palanquin carriers.
A total of 17 Mito rōnin ambushed Ii together with Arimura Jisaemon (Jizaemon Arimura), the lone member of the group who was not from Mito since he was a samurai from Satsuma Domain.
While an attack at the front drew the attention of the guards, a lone assassin fired one shot into the palanquin containing Ii, with a Japanese-made Colt 1851 Navy Revolver, which had been copied from the firearms that Commodore Matthew Perry had given the shogunate as gifts.
Drawing the injured and likely paralyzed Ii out, Arimura decapitated Ii and then performed seppuku.

The conspirators carried a manifesto on themselves, outlining the reason for their act:
While fully aware of the necessity for some change in policy since the coming of the Americans at Uraga, it is entirely against the interest of the country and a stain on the national honour to open up commercial relations with foreigners, to admit foreigners into the Castle, to conclude treaties with them, to abolish the established practice of trampling on the picture of Christ, to allow foreigners to build places of worship for the evil religion, and to allow the three Foreign Ministers to reside in the land ... Therefore, we have consecrated ourselves to be the instruments of Heaven to punish this wicked man, and we have taken on ourselves the duty of ending a serious evil, by killing this atrocious autocrat.

— Manifesto of the Sakuradamon conspirators.

Accounts of the violent event were sent via ship across the Pacific to San Francisco and then sped by pony express across the American West.
 On June 12, 1860, The New York Times reported that Japan's first diplomatic mission to the West received the news about what had happened in Edo.
The assassination dealt a severe blow to the prestige of the Shogunate so that officials refused to admit Naosuke's death for a month, claiming that he was merely injured and was recuperating.
The minister's death was only made official in April when the office of Tairo was declared vacant.
The assassination took place on 24 March 1860, on Double Third Festival where all daimyos stationed in Edo (modern Tokyo) were scheduled to enter Edo Castle for meetings. The assassins attacked Ii's entourage just outside the Castle, near the Sakuradamon (Sakurada Gate) when Ii was reaching the premises. Ii had been warned about his safety, and many encouraged him to retire from office, but he refused, replying that "My own safety is nothing when I see the danger threatening the future of the country".
Ii's entourage was composed of around 60 samurai guards and Ii's palanquin carriers. A total of 17 Mito rōnin ambushed Ii together with Arimura Jisaemon (Jizaemon Arimura), the lone member of the group who was not from Mito since he was a samurai from Satsuma Domain. While an attack at the front drew the attention of the guards, a lone assassin fired one shot into the palanquin containing Ii, with a Japanese-made Colt 1851 Navy Revolver, which had been copied from the firearms that Commodore Matthew Perry had given the shogunate as gifts. Drawing the injured and likely paralyzed Ii out, Arimura decapitated Ii and then performed seppuku.
The conspirators carried a manifesto on themselves, outlining the reason for their act:
While fully aware of the necessity for some change in policy since the coming of the Americans at Uraga, it is entirely against the interest of the country and a stain on the national honour to open up commercial relations with foreigners, to admit foreigners into the Castle, to conclude treaties with them, to abolish the established practice of trampling on the picture of Christ, to allow foreigners to build places of worship for the evil religion, and to allow the three Foreign Ministers to reside in the land ... Therefore, we have consecrated ourselves to be the instruments of Heaven to punish this wicked man, and we have taken on ourselves the duty of ending a serious evil, by killing this atrocious autocrat.
— Manifesto of the Sakuradamon conspirators.
Accounts of the violent event were sent via ship across the Pacific to San Francisco and then sped by pony express across the American West.  On June 12, 1860, The New York Times reported that Japan's first diplomatic mission to the West received the news about what had happened in Edo. The assassination dealt a severe blow to the prestige of the Shogunate so that officials refused to admit Naosuke's death for a month, claiming that he was merely injured and was recuperating. The minister's death was only made official in April when the office of Tairo was declared vacant.
Sakuradamon Incident (1860)