Tsushima, the island between Japan and Korea

Halfway across the strait

The Korea Strait, between Korea and Japan’s islands of Kyushu and Honshu, is split in two by the Tsushima islands. The channel east of them is often called the Tsushima Strait.^[5] Tsushima is closer to Korea than to the rest of Japan: the Korean island of Geoje is less than 50 km away, and from a lookout at the island’s northern end Busan can be seen on clear days.^[2]

More than 90% of Tsushima is mountains, with almost no flat land to farm, so its people lived by fishing and hunting and came to depend on Korea.^[2] The earliest written record of the island, in the 3rd-century Chinese Book of Wei, describes it on the route from the Korean Peninsula to Japan.^[3][2]

A front line

After Tang China and the Korean kingdom of Silla ended Japan’s ambitions on the Korean Peninsula in the mid-600s, Tsushima became a key point in a chain of forts and beacons built against an invasion from the continent, with Kaneda Fort on the island as the first line of defence.^[3]

Kublai Khan’s envoys came to Japan by way of Tsushima, and when Japan ignored them, a Yuan and Goryeo (Korean) fleet appeared off the island in 1274 before attacking Iki. Goryeo forces appeared at Tsushima again in 1281, ahead of the second invasion.^[6] In the 1590s the warlord Toyotomi Hideyoshi used Tsushima as a staging post for two failed invasions of Korea, which cut the island off from its neighbour.^[3]

Tsushima gave its name to another battle in 1905, when Japan’s fleet destroyed the Russian Baltic Fleet in the strait, ending Russia’s hopes of controlling the sea in the Russo-Japanese War.^[4]

Brokers between two countries

In the early 1600s the Sō family, Tsushima’s lords, restored relations with Korea. Through the Edo period they held a monopoly on Japan’s trade with Joseon Korea in return for running diplomacy between the kingdom and the Tokugawa shogunate, and every Korean mission to the shogunate passed through the island.^[3][1]

It took some cunning. When Korea demanded an apology for Hideyoshi’s invasions and the shogunate refused, Tsushima forged one. Once relations were restored it kept its trade and set up a Japanese settlement in Busan again.^[2]

The tie still matters. Korean visitors poured into Tsushima in the 21st century, but when relations between Japan and South Korea soured in 2019, their numbers fell by almost 90%.^[2]