Captain François de Casembroot had been warned. Survivors from a French dispatch vessel, recently attacked while transiting the same passage, had told him explicitly: do not enter the 下関海峡 (Shimonoseki-kaikyō). He went in anyway. The Dutch had maintained a trading relationship with Japan for more than two hundred years, operating through the 出島 (Dejima) trading post in Nagasaki — the only Western nation granted that privilege during the Tokugawa 鎖国 (sakoku), the closed-country policy. Surely two centuries of commercial goodwill would shield a Dutch warship.

It shielded nothing on July 11, 1863.

The HNLMS Medusa, a 16-gun frigate, was transiting the strait when 長州 (Chōshū) shore batteries opened fire without warning. The ship absorbed more than thirty direct hits to the hull. Four sailors died. Sixteen were wounded. The Dutch Consul General, traveling as a passenger, went below decks and stayed there while de Casembroot drove the Medusa through the gauntlet at full speed and out the far side. She was still floating. Barely.

Chōshū was not acting in defiance of the emperor — it was acting on his direct orders. Emperor 孝明 (Kōmei) had issued an 攘夷 (jōi) edict in the spring of 1863: expel the foreign barbarians from Japanese waters. The 尊王攘夷 (sonnō jōi) movement — revere the emperor, expel the barbarians — had been building for a decade, and Chōshū's samurai were among its most fervent adherents. They had already fired on the American merchant vessel Pembroke on June 26, and the French dispatch boat Kien Chan the following day. The Medusa was the third victim in less than three weeks.

The Western powers moved quickly. Five days after the Medusa attack, the USS Wyoming sailed into the strait and attacked Chōshū's fleet, sinking two vessels. But Chōshū's shore batteries held, and the domain's leaders interpreted survival as proof they could withstand Western military force. That interpretation proved catastrophically wrong.

In September 1864, a joint squadron of 17 warships representing Britain, France, the Netherlands, and the United States bombarded the Shimonoseki fortifications for two days. Sixty-two Chōshū cannon were captured and carried off as trophies. The domain was forced to sign the 下関条約 (Shimonoseki Jōyaku) — the Shimonoseki Convention — paying three million dollars in reparations and apologizing for the attacks.

But the humiliation had an unexpected effect. The defeat discredited the 攘夷 (jōi) hardliners within Chōshū and brought reformist samurai — including Kidō Takayoshi and Takasugi Shinsaku — to power. They formed the 奇兵隊 (Kiheitai), a mixed-class militia armed with Western weapons, and pivoted from 'expel the barbarians' to 'adopt their technology and overthrow the shogunate.' The Satsuma-Chōshū Alliance followed in 1866. The Meiji Restoration followed in 1868.