The Amagi class (天城型, Amagi-gata) was a series of four battlecruisers planned for the Imperial Japanese Navy (IJN) as part of the Eight-eight fleet in the early 1920s.

The Amagi design was essentially a lengthened version of the Tosa-class battleship, but with a thinner armored belt and deck, a more powerful propulsion system, and a modified secondary armament arrangement.

Akagi was reconstructed as an aircraft carrier and served with distinction as part of the Kido Butai during the Second World War, participating in the Japanese attack on Pearl Harbor before being sunk at the Battle of Midway.

The design staff intended to use turbines, which were to be powered by 19 Kampon water-tube boilers, eleven of which were oil-fired, while the other eight were to have mixed oil and coal for fuel.

However, naval technology was changing; older battleships, including all of Japan's battleships in commission or under construction,[A 2] were quickly rendered obsolete with the commissioning of HMS Dreadnought (hence the terms dreadnought and pre-dreadnought), and armored cruisers were seemingly useless in the face of the new battlecruisers being laid down by Great Britain and Germany.

This meant that any naval planner aiming for an eight-eight fleet would have to call for seven more battleships and four more battlecruisers[5] at a time when Japan was trying to weather a worldwide economic depression.

In late 1917, the Navy proposed to expand the eight-four plan by adding two more battlecruisers; this was approved, and two more Amagi-class ships were ordered.

However, having eight 41-centimetre gun ships (four battleships and four battlecruisers) on order put an enormous financial strain on Japan, which was spending about a third of its national budget on the Navy.

The massive size and scale of its building program was rapidly driving up the cost of naval construction and armament.

[8] The Washington Naval Treaty, signed in February 1922, greatly reduced the tonnage allowed for capital ships in the signatory nations.

However, the Americans also had the same problem when designing a conversion of their Lexington class, so an exception, spearheaded by US Assistant Secretary of the Navy Theodore Roosevelt Jr., was added to the treaty that gave the five signatories the option of converting up to two capital ships that were under construction to 33,000-ton aircraft carriers.

[1] The incomplete Tosa-class battleship Kaga, on which work had stopped on 5 February 1922, was reordered as a carrier to replace Amagi.

[16] Nagumo's Kido Butai—composed of the carriers Akagi, Kaga, Hiryū, Sōryū, Shōkaku, and Zuikaku, supported by escorts—launched two waves of airstrikes on the American base at Pearl Harbor in a devastating surprise attack.

[21] Akagi and the carriers Hiryū and Sōryū were sent in March 1942 with a mixed force of battleships, cruisers, and destroyers to the Indian Ocean to engage the British fleet there and to support planned attacks on Ceylon.

[23] In late May 1942, in an effort to draw out and destroy the elusive American carriers, Japanese forces organized attacks on the Aleutian Islands in Alaska and Midway Atoll in the Western Pacific.

[24] Nagumo, aboard Akagi, led Kaga, Sōryū, and Hiryū and the support ships of the First Carrier Striking Force to Midway.

[25][26] In the initial attack, Japanese planes neutralized a small force of fighter aircraft and inflicted heavy damage to American installations.

[29] American dive-bombers, arriving late after difficulty locating the fleet, soon landed fatal strikes on Akagi, Kaga, and Sōryū.