On June 25, 1863, Choshu (長州, Chōshū) Domain warships fired without warning on the American merchant steamer Pembroke as it anchored outside the Shimonoseki (下関, Shimonoseki) Strait. The crew of a Choshu vessel reportedly taunted the Americans with the battle cry of the age: "Sonnō jōi!" (尊王攘夷, sonnō jōi) — "Revere the Emperor, expel the barbarians!" The Pembroke slipped away through the Bungo (豊後, Bungo) Strait with only slight damage and no casualties.

The very next day, June 26, Choshu shore batteries turned on a warship. The French naval dispatch steamer Kienchang (also rendered Kien-Chang) was riding at anchor outside the strait when Choshu opened fire. The vessel was struck and at least one French sailor was wounded. That the target was a military vessel was significant: France now had a direct grievance of a different order than the American merchant it had heard about the day before.

Choshu's daimyo (大名, daimyō), Mori Takachika (毛利敬親, Mōri Takachika), was 44 years old and acting on an explicit imperial command. Earlier that year, Emperor Komei (孝明天皇, Kōmei Tennō) had issued his "Order to expel barbarians" (攘夷実行の勅命, jōi jikkō no chokumiyo) — a remarkable departure from two centuries of imperial practice, during which the court had stayed entirely out of foreign policy. The order placed the Tokugawa shogunate (徳川幕府, Tokugawa bakufu) in an impossible position. The shogunate had signed the "unequal treaties" with Western powers and had every reason to avoid a military confrontation with European navies. But the shogunate also could not openly defy an imperial command. Choshu, the most militantly anti-foreign domain in Japan, resolved the ambiguity by simply obeying the Emperor and ignoring the shogunate.

The attacks continued. On July 11, the Dutch warship HNLMS Medusa entered the strait. The Dutch had maintained Japan's sole Western trading relationship for more than 220 years — a relationship that had survived every shift in Japanese politics since 1641. It made no difference. Choshu batteries struck the Medusa more than thirty times, killing or wounding nine Dutch sailors. Three Western nations had been attacked in three weeks, and the shogunate had done nothing to stop it.

Western retaliation was swift. On July 16, 1863, the USS Wyoming sailed alone into the strait and fought Choshu warships for nearly two hours, sinking two enemy vessels and inflicting approximately 40 casualties at a cost of 4 American dead and 7 wounded. On July 20, French forces landed at Shimonoseki and burned Choshu shore installations. But the batteries remained largely intact. The decisive blow came in August 1864, when a combined fleet of 17 warships from Britain, France, the Netherlands, and the United States — commanded by British Admiral Sir Augustus Kuper — bombarded Shimonoseki for two days and finally destroyed Choshu's coastal defenses. Takasugi Shinsaku (高杉晋作, Takasugi Shinsaku), then 25 years old, negotiated the peace on September 8, 1864. The resulting Shimonoseki Convention imposed a $3 million indemnity on Japan — to be paid by the shogunate, not Choshu, a final humiliation for the central government.

The aftermath transformed Choshu in ways no one had predicted. Defeated by Western firepower, Choshu's reformers — Takasugi, Ito Hirobumi (伊藤博文, Itō Hirobumi), Yamagata Aritomo (山県有朋, Yamagata Aritomo) — concluded that Japan must master Western military methods to survive. Takasugi formed the Kiheitai (奇兵隊, kiheitai), a non-samurai conscript militia modeled on Western infantry. The slogan "sonnō jōi" (expel the barbarians) was replaced by "fukoku kyōhei" (富国強兵, fukoku kyōhei) — "rich country, strong army." Choshu, the domain that had fired on Western ships, became the domain that built Japan's modern army. Within five years, the shogunate was gone.