Tsushima Fortress

Tsushima Fortress (対馬要塞, Tsushima Yōsai) was one of the most extensive coastal defense systems in the Japanese Empire, spanning the entirety of Tsushima Island — a 70-kilometer-long archipelago positioned strategically in the Korea Strait between Japan and Korea. The fortress was designed to block enemy fleets from passing through the strait and to protect the Imperial Japanese Navy's Takeshiki Naval Port (竹敷要港) in Aso Bay (浅茅湾), which served as a critical forward operating base for operations toward the Korean Peninsula and the Asian mainland.

Construction began in April 1887 (Meiji 20), making Tsushima the second fortress to be built in Japan, after the Tokyo Bay defenses. The impetus was Russia's aggressive southward expansion and fears of a naval assault through the Korea Strait. The fortress was built in three major phases spanning over 50 years.

Phase 1 (Meiji Period, 1887–1888): Four batteries were completed around Aso Bay by October 1888, well before the First Sino-Japanese War — Imozaki Battery (28cm howitzers ×4), Ohihira Low Battery (12cm cannons ×4), Nukue Battery (12cm cannons ×4), and Oishiura Battery (28cm howitzers ×6). These initial positions defended the entrance to Aso Bay and the Takeshiki naval facilities.

Phase 2 (Meiji Period, 1898–1903): With the Russo-Japanese War looming, construction intensified dramatically. Between 1898 and 1903, batteries were expanded to cover three bays — Aso Bay, Miura Bay (三浦湾), and Shūshi Bay (舟志湾). Major additions included the Shijuyatani Battery (28cm howitzers ×6), Joyama Battery (28cm howitzers ×4), Himegamiyama Battery (28cm howitzers ×6), and multiple positions at Neo and surrounding headlands. By March 1903, twelve batteries and redoubts had been completed, armed primarily with 28cm howitzers and 12cm cannons. During the Battle of Tsushima on May 27–28, 1905, the fortress stood ready as Admiral Togo's Combined Fleet annihilated the Russian Baltic Fleet in the strait waters directly adjacent to these fortifications.

Phase 3 (Taisho–Showa Period, 1924–1939): After a pause following the Russo-Japanese War, modernization resumed in 1924 with longer-range weapons to counter advances in naval gunnery. The crowning achievement was the Toyo Battery (豊砲台), completed in March 1934 at the northern tip of Tsushima. It mounted two 45-caliber 40cm (16-inch) guns in a twin turret salvaged from the battleship Tosa, whose construction had been cancelled under the Washington Naval Treaty of 1922. These were among the largest fortress guns in the world at the time. Additional batteries were built in this period, including Tatsunozaki Batteries No. 1 (1924–1929) and No. 2 (1933–1936), with 30cm cannons, and, between 1934 and 1939, seven coastal batteries armed with 15cm cannons at points around the island including Saozaki, Tsutsuzaki, Unijima, Gozaki, Osaki, Nishidomari, and Takesaki.

During World War II, the fortress continued its mission of strait defense, though by then the primary threat had shifted from surface naval attack to submarine and aerial warfare. The Toyo Battery's massive 40cm guns, with a range exceeding 30 kilometers, served as a powerful deterrent — though they were never fired in combat, earning them the nickname "the guns that never fired" (撃たざる巨砲). The Tsushima Fortress Command (対馬要塞司令部) coordinated with the Tsushima Fortress Heavy Artillery Regiment (対馬要塞重砲兵連隊) and worked alongside Imperial Navy units to maintain the island's defenses throughout the war.

Following Japan's surrender in August 1945, American occupation forces systematically dismantled the fortress installations, destroying gun emplacements and removing armaments. Today, 31 battery sites remain across the island in varying states of preservation. The Toyo Battery is the best-preserved, with its massive underground gun chambers, ammunition magazines, and mechanical rooms still intact and accessible to visitors (interior lighting available via switch). The Joyama Battery retains well-preserved gun emplacements and underground structures. The Saozaki Battery site has been converted into a park with a lighthouse marking Japan's northwesternmost point, from which Busan, South Korea is visible on clear days. The Himegamiyama Battery offers spectacular coastal views alongside its ruins. Several sites including Neo and Gosaki remain restricted due to active Japan Self-Defense Force operations on the island.