For 263 years, the Tokugawa shogunate (徳川幕府, Tokugawa bakufu) had been the undisputed master of Japan. No domain had successfully defied its military authority. When Chōshū (長州) domain — a fiercely anti-shogunate han (藩) on the western tip of Honshū — refused to submit after years of escalating confrontation, the shogunate's response was overwhelming force. On June 7, 1866, the shogunate's navy opened the Second Chōshū Expedition (第二次長州征伐, Daini-ji Chōshū seibatsu) with a bombardment of the island of Suō-Ōshima (周防大島). The message was unmistakable: defiance would be crushed.

Chōshū did not submit. What the shogunate's commanders encountered was something the Japanese military had never faced before: the Kiheitai (奇兵隊), Takasugi Shinsaku's (高杉晋作) irregular militia. Takasugi was 26 years old, a Chōshū samurai who had studied in Edo (江戸) and witnessed the arrival of Western warships firsthand. He had drawn a conclusion that struck most of the samurai class as scandalous: the old hereditary warrior system could not fight a modern war. In 1863, he founded the Kiheitai — a volunteer force that recruited farmers, merchants, craftsmen, and townspeople alongside samurai. All of them trained together. All of them drilled with imported Enfield rifles. All of them operated under the same tactical doctrine of speed, flexibility, and skirmishing rather than the slow, formation-based methods that still dominated shogunate thinking.

By opening the campaign with a naval bombardment, the shogunate expected to establish dominance quickly. Instead, Chōshū held Suō-Ōshima, and the campaign ground into a four-front war. On each front — Suō-Ōshima, Ōshima, the Kokura domain border, and the Hamada domain — the shogunate's forces failed to achieve their objectives. Chōshū's defenders, numbered at approximately 3,500 to 4,000 troops, outfought an attacking force that should have been far superior in size and institutional authority.

Beneath the military failure lay a diplomatic earthquake. The Satsuma (薩摩) domain — Chōshū's longtime rival in western Japan, and previously one of the forces that had humiliated Chōshū in earlier confrontations — refused to contribute troops to the expedition. This was not an accident. In January 1866, just months before the campaign began, Satsuma and Chōshū had secretly concluded the Satchō Alliance (薩長同盟, Satchō Dōmei) — a historic agreement between two domains that had been enemies. They had concluded, independently and then together, that the shogunate was finished and that the future of Japan lay elsewhere. Their alliance held. Every domain watching the expedition understood what Satsuma's refusal meant.

Shogun Tokugawa Iemochi (徳川家茂) had traveled to Osaka Castle to oversee the campaign personally. He died there on August 29, 1866, at the age of 20. The cause was beriberi — a disease caused by thiamine deficiency, common among those whose diet relied heavily on polished white rice. The most powerful man in Japan, the figurehead of a government claiming dominion over every daimyo (大名) and domain, was undone by a vitamin deficiency while his army suffered defeat on every front around him. His death gave the shogunate a face-saving pretext: the expedition was suspended to mourn the shogun. The ceasefire was negotiated by Tokugawa Yoshinobu (徳川慶喜), who became the new — and last — shogun. But no political framing could undo what had happened militarily.

Takasugi Shinsaku, the man most responsible for Chōshū's victory, never saw its ultimate consequence. He died of tuberculosis on May 17, 1867, at the age of 27 — less than a year after the expedition failed, and nearly a year before the Meiji Restoration he had helped make possible. He had dissolved, in practice, the class distinction that defined Japanese society, by proving that a farmer with a rifle and proper training could fight as well as a samurai. The weapon that stopped the shogunate was not just the Enfield rifle; it was the idea behind the Kiheitai.