They called themselves the Emperor's loyal servants. By nightfall, they had set his capital on fire.

It was the 19th day of the 7th month of 元治元年 (Genji gannen) — the traditional Japanese lunisolar calendar date, corresponding to August 20, 1864 in the Western calendar — and the confrontation had been building for eleven days. On July 8, the 新選組 (Shinsengumi, Shinsengumi) — the shogunate's elite police force — had burst into the 池田屋 (Ikedaya, Ikedaya) Inn in Kyoto and cut down eight anti-shogunate activists, arresting twenty-three more. Among the survivors were men from 長州藩 (Chōshū-han, Choshu domain). They went home and came back with an army.

Approximately 3,000 Choshu samurai marched on the 蛤御門 (Hamaguri Gomon, Hamaguri Gate) of the Kyoto Imperial Palace under the banner of 尊王攘夷 (sonnō jōi) — revere the Emperor, expel the barbarians. The irony was total and did not occur to them: they were attacking the palace of the emperor they claimed to be defending.

薩摩 (Satsuma, Satsuma) and 会津 (Aizu, Aizu) troops held the gate. The gate still stands in Kyoto today, named for the clam-shaped metal fittings on its doors. The battle was fierce and short. Choshu commanders 来島又兵衛 (Kijima Matabei, Kijima Matabei) and 国司信濃 (Kunishi Shinano, Kunishi Shinano) were killed in the fighting. 真木和泉 (Maki Izumi, Maki Izumi) and 久坂玄瑞 (Kuzaka Genzui, Kuzaka Genzui) committed 切腹 (seppuku). The Choshu assault shattered.

As the surviving Choshu forces broke and retreated, fires began. Whether they were set deliberately as a final act of defiance or spread from the chaos of battle, the outcome was the same. The どんどん焼け (dondon yake) — named for the rumbling, roaring sound of a great conflagration — swept across the ancient city. Approximately 28,000 structures burned. London's Great Fire of 1666, for comparison, destroyed about 13,200 houses. Kyoto, the imperial capital since 794, had not seen destruction on this scale in living memory.

The shogunate launched the First Chōshū Expedition in September 1864, forcing Choshu to formally submit, execute its most radical leaders, and disband its irregular forces. It looked like the end of the movement.

It was not. Choshu's defeat radicalized the domain further. By 1866, Choshu and Satsuma — the very forces that had held the gate against them in 1864 — had formed a secret alliance. By 1868, they had overthrown the Tokugawa shogunate. The men who fought on opposite sides of the 蛤御門 (Hamaguri Gomon) became, together, the founders of Meiji Japan.