The Tsushima Strait is the broader eastern channel to the east and southeast of Tsushima Island, with the Japanese islands of Honshū to the east and northeast, and Kyūshū and the Gotō Islands to the south and southeast. It is narrowest south-east of Shimono-shima, the south end of Tsushima Island proper, constricted there by nearby Iki Island, which lies wholly in the strait near the tip of Honshū. South of that point Japan's Inland Sea mingles its waters through the narrow Kanmon Strait between Honshū and Kyūshū, with the those of the Tsushima Strait, making for some of the busiest sea lanes in the world.
The strait measures approximately 100 kilometres along Tsushima Island and is about 65 kilometres wide at its narrowest. The strait has a depth of about 90 metres and is bounded by the Tsushima Islands to the west through north (of Gotō Islands). Nearby Iki Island lies in the strait about 50 kilometres towards Kyūshū from the southern tip of Kamino-shima (South Island).
The Tsushima Current, a warm branch of the Kuroshio (Japan Current) passes through the strait. Originating along the Japanese islands, this current passes through the Sea of Japan then a branch eventually flowing into the Pacific Northern Pacific Ocean via the Tsugaru Strait south of Hokkaidō. Another branch continues far northward and divides along either shore of Sakhalin Island; eventually flowing into the Sea of Okhotsk via the La Perouse Strait north of Hokkaidō and via the Strait of Tartary into the Sea of Okhotsk north of Sakhalin Island near Vladivostok.
The Tsushima Current brings rich fisheries resources from the East China Sea into the Sea of Japan, like Japanese amberjack, Japanese horse mackerel, but these days also brings disaster like mass aggregation of gigantic Nomura's jellyfish and waste from countries along the course of the current.
A commercial ferry service operates between Shimonoseki at the western tip of Honshū and Busan (aka Pusan), South Korea. Another operates between Shimonoseki and Tsushima Island. The Cities of Kitakyushu (Kyūshū) and Shimonoseki (Honshū) are joined by an ocean-spanning bridge across the Kanmon Strait joining those cities with Nagasaki, which latter city serves as prefecture-level capital and administers both Tsushima and Iki Island. Kanmon Strait lies approximately 85 miles (135 km) due east of the center of Tsushima Island, while Nagasaki city proper lies about 100 miles (165 km) to the south-south-east of the southern tip of the island.
The straits also served as a migration or an invasion path, in both directions. For example, archeologists believe the first Mesolithic migrations (Jōmon) traveled across to Honshū around the 10th century BC, supplanting Paleolithic people that arrived over 100,000 years ago, when there was no water between the Asian continent and Japan (it was land). Immigrants from Goguryeo, Gaya Confederacy, and Baekje also contributed to waves of immigrants arriving in Kyūshū, although who, when, and how many exactly is a matter of intense debate. Buddhism, along with Chinese writing, was initially transmitted from Baekje to Japan in the 5th century by way of the straits as well. Iki to Kamino-shima, the southern end of the large island of Tsushima, is about 50 kilometres. Busan (Korea), to the Northern tip of Tsushima, about the same across the Korea Strait. These were tremendous distances to attempt in small boats over open seas.
The Mongolian invasion of Japan crossed this sea and ravaged the Tsushima Islands before the kamikaze (神風) – translated as "divine wind" – a typhoon that is said to have saved Japan from a Mongol invasion fleet led by Kublai Khan in 1281. Military commander Toyotomi Hideyoshi and loyal allies of Western Japan (but not the East) led an invasion of Korea by way of the straits in the late 1500s, and more recently, the Japanese Imperial Army had attacked Korea and beyond.
But the reason the strait is famous is that one of the most decisive naval battles of modern times, the Battle of Tsushima, fought on May 27 and May 28, 1905 took place there due east of the north part of Tsushima and due north of Iki Island (shown in red on the second map) between the Japanese and Russian navies in 1905; the Russian fleet was virtually destroyed by the Japanese. This decisive result was to affect Naval planners and Fleet Admirals for the next forty years with a type of tunnel vision such that national and naval leaders were continuallly looking for the chance or to create that set of circumstances which would lead to a similar decisive major fleet engagement— while ignoring objective realities such as the new and eventually overwhelming ability of air power to devastate and neutralize the big gun ship. Even brilliant strategists such as Britain's Admiral Sir John Jelicoe (Battle of Jutland) and air power enthusiastists and supporters like Japan's Combined Fleet Commanding Fleet Admiral Isoroku Yamamoto (Battle of Midway), or a tactician like American Vice-Admiral William 'Bull' Halsey (Battle of Leyte Gulf) fell prey to the 'Big Fleet Battle Theory', consequently ignoring other tactical realities with an over focus on 'The Big Score'; an idealization which eluded all.