Snow was falling on Edo when Ii Naosuke's procession formed up and moved toward the castle. It was the morning of the 雛祭り (Hinamatsuri), the Doll Festival — the third day of the third month — when every daimyo in the city was expected to appear at the shogun's court to pay ceremonial respects. The streets were quiet in the way that heavy snow makes streets quiet. Ii traveled in a lacquered palanquin, surrounded by sixty retainers. He had been warned, more than once, that men wanted to kill him. He had posted additional guards. He had made the same trip before.

Ii Naosuke (井伊直弼, 1815–1860) was the 大老 (Tairō), the Great Elder — the highest office in the Tokugawa shogunate below the shogun himself. The position had existed for emergencies, and his two years in it had been nothing but. He had signed the 日米修好通商条約 (Nichi-Bei Shūkō Tsūshō Jōyaku), the Harris Treaty opening Japan to American trade, without imperial sanction — a political earthquake. He had then moved to silence the protest by arresting, exiling, or executing over a hundred critics, scholars, and rival lords in what became known as the 安政の大獄 (Ansei no Taigoku), the Ansei Purge. Among those he imprisoned was Tokugawa Nariaki, the powerful lord of Mito domain and father of the future last shogun — a man whose followers had spent generations studying Japanese history and had concluded that the emperor, not the shogun, was Japan's rightful sovereign.

That ideology — 尊王攘夷 (sonnō jōi), revere the emperor, expel the barbarians — had been building pressure for years. Ii's purge turned pressure into fury.

At the 桜田門 (Sakuradamon), the principal outer gate of Edo Castle, eighteen men were waiting in the snow. Seventeen were 浪人 (rōnin) from Mito — former retainers who had deliberately resigned their service to become masterless specifically so that this act could not be blamed on Mito domain as a whole. The eighteenth was Arimura Jizaemon, a Satsuma samurai who had no Mito connection and left behind a written testament explaining, in measured prose, why Ii had to die.

They had rehearsed the approach. They moved as the procession reached the gate — simultaneously, from multiple directions. The snow-soaked oilskin rain covers that Ii's guards had tied over their sword hilts to protect the steel from moisture made it nearly impossible to draw in time. One assassin pulled out a pistol — some accounts describe it as a Japanese-made reproduction of a Western revolver, a detail contemporary observers found grimly ironic, given that the men holding it claimed to be fighting Western influence — and fired into the palanquin, wounding Ii inside.

Arimura Jizaemon pulled the palanquin door open. Ii Naosuke, already bleeding, was dragged into the road in front of his own castle gate. Arimura beheaded him in the snow. He then performed 切腹 (seppuku) on the spot and died there, beside the man he had killed.

Most of the other attackers were wounded or killed in the fighting. Those who survived were captured in the days that followed and were eventually executed.

The shogunate's response was, in its own way, as significant as the assassination itself. Officials gathered up the body, sealed the information inside the government, and for the next four weeks maintained the fiction that Ii Naosuke was ill and recovering at his estate. Japan's most powerful official had been killed in broad daylight at the front gate of the shogun's castle, and the government could not bring itself to admit it. The office of 大老 (Tairō) was declared vacant in April 1860 — at which point the death became official, without the word 'assassination' ever being used in the announcement.

The cover-up revealed more than the murder did. A government that controlled the country could absorb the loss of one man. A government that needed to hide the loss of one man was already losing control of the country.

The Sakuradamon Incident — 桜田門外の変 (Sakuradamon-gai no Hen) — unleashed eight years of accelerating crisis. Ii's purge had created martyrs; his assassination created heroes. Men who had been afraid to act watched eighteen 浪人 (rōnin) walk into the most guarded place in Japan and succeed, and took note. A wave of political violence — assassinations of shogunate officials, attacks on foreign diplomats, incidents in Kyoto and the countryside — followed throughout the 1860s. The shogunate tried repression, then concession, then alliances, then war, and lost every round. In 1868, the 明治維新 (Meiji Ishin), the Meiji Restoration, ended the Tokugawa shogunate permanently. The era that Ii had tried to defend with imprisonment and executions outlasted him by eight years.