Ganghwa Island sits at the mouth of the Han River, guarding the sea approach to Seoul. By 1875, it had already earned a grim reputation as a flashpoint between Korea and the outside world. In 1866, French forces briefly occupied the island during a punitive expedition triggered by the execution of French missionaries. Five years later, in 1871, an American naval expedition attacked the island's fortifications in what Koreans call the Sinmiyangyo -- a battle that left hundreds of Korean defenders dead but failed to pry open the kingdom's doors. The Joseon court's policy of isolation, enforced under the regent Heungseon Daewongun, had held against Western powers. Japan would try a different approach.

On the morning of September 20, 1875, the Japanese gunboat Un'yo, under Commander Inoue Yoshika, appeared off the coast of Ganghwa Island. The official purpose was surveying Korean coastal waters. A landing party went ashore to request water and provisions -- a routine enough pretext, though the crew carried modern rifles. When Korean shore batteries opened fire on the Un'yo, the Japanese response was immediate and devastating. The gunboat bombarded the fortifications, then landed troops who torched houses and engaged Korean soldiers. The technological gap was brutal: Japanese marines armed with modern rifles faced Korean defenders still carrying matchlock muskets, weapons that had scarcely changed since the sixteenth century. Thirty-five Korean soldiers died. Japan suffered two wounded.

News of the clash did not reach Tokyo until September 28, but the Japanese government moved quickly. Gunboats were dispatched to Busan to protect Japanese residents, and deliberations began over how to leverage the incident. The playbook was familiar -- it was the same strategy Commodore Perry had used to open Japan itself just two decades earlier. Japan blockaded the area and demanded an official apology. The Joseon government, weakened and isolated, had few options. In February 1876, Japan sent the Kuroda mission, backed by warships, to negotiate. The result was the Treaty of Ganghwa, signed on February 27, 1876, which forced Korea to open three ports to Japanese trade and recognize Japan as an equal sovereign power independent of China's tributary system.

The Treaty of Ganghwa marked the beginning of the end for Korea's centuries of deliberate isolation. The treaty's provisions were modeled on the unequal treaties that Western powers had imposed on Japan and China -- extraterritorial rights, forced port openings, favorable trade terms. Japan, having been on the receiving end of such treaties barely a generation earlier, now imposed them on its neighbor. In addition to the 35 Korean soldiers killed, 16 naval personnel were captured, and Japanese forces looted weapons from the fortifications. The incident set in motion a chain of events that would lead to the First Sino-Japanese War of 1894, the Russo-Japanese War of 1904, and ultimately Japan's annexation of Korea in 1910. What began as a dispute over two Chinese characters ended with the erasure of Korean sovereignty entirely.