Anti-Aircraft Guns Installed on Sarushima

Sarushima is an uninhabited island about 1.7 km off Yokosuka and the largest natural island in Tokyo Bay. The Tokugawa shogunate built a battery there in 1847, and to defend the entrance to the bay the Meiji government had the Army construct a new battery on the island from 1881, completed in 1884, as part of the Tokyo Bay Fortress system. The works consist of open cuttings and tunnels dug into the island's bedrock and lined with red brick laid in Flemish bond, with barracks and magazines opening off the cutting.

The battery was damaged in the Great Kanto Earthquake of 1923 and, according to a travel guide, was struck from the fortress list in 1925, after which the Navy used the island for the defence of Yokosuka naval port. As Japan entered World War II in 1941, anti-aircraft guns were installed on Sarushima to strengthen the air defense network protecting Tokyo Bay and the strategically vital Yokosuka Naval Base; the same guide gives the armament as four 8 cm high-angle guns and two twin 12.7 cm high-angle mounts (four guns), with searchlights and an air-defence command post at the centre of the island. The island, already part of the Tokyo Bay Fortress system since the 1880s, was thus adapted from a purely coastal defense role to include anti-aircraft capability, and circular concrete gun emplacements of the Showa period were added above the Meiji brickwork.

Sarushima remained an active military installation throughout the war until Japan's surrender in August 1945, when American occupation forces seized the island and removed all functional military equipment. It was returned to Japan in 1961; a ferry had run since 1947 and a bathing beach opened in 1957. Yokosuka City took over its management in 1995 and received the island without charge in 2003, developing it as Sarushima Park.

On 10 March 2015 the Sarushima battery and the Chiyogasaki battery were designated a National Historic Site as the 'Tokyo Bay Fortress Sites', the first designation of military works built after the Meiji Restoration. Visitors today reach the island by ferry from Yokosuka and can walk past the brick tunnels, barracks and magazines and the remains of the Showa-period anti-aircraft emplacements.