Tokyo Bay Fortress

The defence of the entrance to Tokyo Bay began under the Tokugawa shogunate, which built batteries off Shinagawa and along the Uraga Channel in response to foreign ships; Sarushima received its first battery in 1847. The Meiji government, concerned to protect the capital and the Yokosuka shipyard, began building Western-style batteries at Kannonzaki on the Miura Peninsula in 1880, the start of modern fortress construction in Japan, and the brick fortifications on Sarushima were completed in the 1880s. The Tokyo Bay Fortress Command was established in 1895 at Yokosuka to control the batteries and troops on both shores. Under the Fortified Zone Law of 1899, surveying, photography, sketching and building around the fortress were strictly controlled.

The Tokyo Bay Fortress (東京湾要塞) was a comprehensive coastal defense system established by the Imperial Japanese Army to protect the entrance to Tokyo Bay. Spanning both sides of the bay — the Miura Peninsula in Kanagawa and the Boso Peninsula in Chiba — the system comprised 32 batteries and sea forts (海堡). The concept was crossfire from batteries at Kannonzaki, Chiyogasaki and Hashirimizu and around Cape Futtsu, with three artificial islands built on the shoals between them to close the channel. No. 1 Sea Fort was completed in 1890, No. 2 in 1914 and No. 3 in 1921, the latter two after decades of marine engineering in deep, fast water. Major installations included Sarushima Battery (the first, dating to 1847), Chiyogasaki Battery, the three Kaiho sea forts, and batteries at Kannon-zaki and Natsushima.

The Great Kantō Earthquake of 1923 severely damaged No. 2 and No. 3 Sea Forts; No. 3 subsided about 4.8 metres, a third of it under water, and was beyond practical repair. As naval gun ranges grew, the defensive line moved out to the mouth of the bay: heavy gun turrets removed from capital ships scrapped under the Washington Naval Treaty were emplaced as shore batteries at Jōgashima at the tip of Miura and at Sunosaki and Taibusa on the southern Bōsō coast.

The fortress system was active from the late Edo period through World War II. In the Pacific War it guarded the bay entrance, contributed to air defence and in 1945 was incorporated into preparations for the decisive battle on the home islands, but no Allied fleet forced the bay and it saw no combat; Japan's surrender was signed aboard USS Missouri in the bay on September 2, 1945. The batteries were disarmed after the war. The Sarushima and Chiyogasaki battery sites were designated a National Historic Site in 2015, the first such designation for military works built after the Meiji Restoration, while No. 3 Sea Fort, a hazard to shipping, was removed in the 2000s and parts of it preserved on land.