The Tosa-class battleships (土佐型戦艦, Tosa-gata Senkan)[A 1] were two dreadnoughts ordered as part of the "Eight-Eight" fleet for the Imperial Japanese Navy (IJN) during the early 1920s.

The carrier supported Japanese troops in China during the Second Sino-Japanese War of the late 1930s, and took part in the attack on Pearl Harbor on 7 December 1941 and the invasion of Rabaul in the Southwest Pacific in January 1942.

The IJN believed that a modern battle fleet of eight battleships and eight armored cruisers was necessary for the defense of Japan; the government ratified that idea in 1907.

[1] This policy was the genesis of the Eight-Eight Fleet Program, the development of a cohesive battle line of sixteen capital ships less than eight years old.

It added an extra twin main-gun turret, using space and weight made available by the reduction of the number of boilers from 21 to 12 while the power remained the same.

He reduced the secondary armament from 20 guns to 16; they were moved up a deck to improve their arcs of fire and their ability to shoot during heavy weather.

[8] The Tosa-class ships were intended to be armed with a main battery of ten 45-caliber 41-centimeter (16.1-inch) guns in five twin turrets, four of which were superfiring fore and aft.

[11] The ships' secondary armament of twenty 50-caliber 3rd Year Type 14-centimeter (5.5-inch) guns would have been mounted in casemates, 12 on the upper sides of the hull and eight in the superstructure.

This was backed by a torpedo bulkhead also made up of three 25 mm (0.98 in) layers of HTS and angled outwards to meet the base of the waterline belt.

[19] After being stricken on 1 April 1924,[8] Tosa's guns were turned over to the Imperial Japanese Army for use as coastal artillery; two of her main-gun turrets were installed on Tsushima Island and near Busan, Korea.

[21] Tosa's incomplete hull was used to test her armor scheme against long-range naval gunfire, aerial bombs, mines, and torpedoes.

After the conclusion of the tests, the ship was scuttled by opening her Kingston valves on 9 February 1925 in 650 m (2,130 ft) of water in the Bungo Channel after the demolition charges failed to detonate.

[22] The battlecruiser Amagi, which was being converted to an aircraft carrier under the terms of the treaty, was wrecked in the Great Kantō earthquake in 1923 and rendered unusable.

As a result, Kaga, which was originally slated to be scrapped under the terms of the Washington Naval Treaty (Chapter I, Article IX),[8] was converted in Amagi's stead.

[25] Kaga was provided with a heavy gun armament in case she was surprised by enemy cruisers and forced to give battle, but her large and vulnerable flight deck, hangars, and other features made her more of a target in any surface action than a fighting warship.

[26] The ship was armed with ten 20 cm/50 3rd Year Type guns: one twin-gun turret on each side of the middle flight deck and six in casemates aft.

[27] In 1933–35 Kaga was rebuilt to increase her top speed, improve her exhaust systems, and adapt her flight decks to more modern, heavier aircraft.

[28] Kaga's aircraft first supported Japanese troops in China during the Shanghai Incident of 1932 and participated in the Second Sino-Japanese War in the late 1930s.

[29] With five other fleet carriers, she took part in the Pearl Harbor raid in December 1941 and the invasion of Rabaul in the Southwest Pacific in January 1942.

The following month her aircraft participated in a combined carrier airstrike on Darwin, Australia, helping secure the conquest of the Dutch East Indies by Japanese forces.